JHR 95: 215-230 (2023) yee eS,” JOURNAL OR. SMe wwnteeicatiante

doi: 10.3897/jhr.95.98260 RESEARCH ARTICLE () Hymenopter a
g

https://jhr.pensoft.net The Inarasional Society of Hymenopreriss, RESEARCH

Morphological specialisation for primary
nectar robbing in a pollen specialist mining bee
(Hymenoptera, Andrenidae)

Andreas Miiller'!, Paul Westrich?

| ETH Zurich, Institute of Agricultural Sciences, Biocommunication and Entomology, Schmelzbergstrasse 9/
LFO, 8092 Zurich, Switzerland 2. Raichbergstrasse 38, 72127 Kusterdingen, Germany

Corresponding author: Andreas Miiller (andreas.mueller@usys.ethz.ch)

Academic editor: Jack Neff | Received 1 December 2022 | Accepted 30 January 2023 | Published 17 February 2023
/ttps://z00 bank. org/84C BOF4F-360C-4E78-8432-17196C6D620F

Citation: Miiller A, Westrich P (2023) Morphological specialisation for primary nectar robbing in a pollen specialist
mining bee (Hymenoptera, Andrenidae). Journal of Hymenoptera Research 95: 215-230. https://doi.org/10.3897/
jhr.95.98260

Abstract

The European mining bee species Andrena lathyri (Andrenidae) is a narrow specialist of flowers of Lathyrus
and Vicia (Fabaceae), from which both females and males gain nectar by primary nectar robbing. Both
sexes are equipped with a unique proboscis, which is much longer and more strongly angled than in most
other Andrena bees including the most closely related species. The comparison between the shape of the
proboscis and the interior of the host flowers combined with field observations revealed that the special-
ised mouthparts of A. lathyri precisely correspond to the dimensions of the flower interior and the position
of the nectary, representing one of the few known examples of a morphological adaptation to primary nec-
tar robbing in bees. For nectar uptake, the bee’s head is inserted laterally under the standard petal before
it is moved towards the flower base, thereby slitting the calyx longitudinally to a depth necessary to reach
the nectary from inside the flower with the specialised proboscis. Nectar-robbing individuals of A. lathyri
are able to adapt their behaviour to the different calyx lengths of their host flower species by slitting the
calyx over varying distances. Except for the slit in the calyx, primary nectar robbing by A. /athyri does not

damage any flower parts allowing for normal fruit development.

Keywords
Anthophila, Apiformes, bee-flower relationships, Fabeae, Lathyrus pratensis, Taeniandrena, Vicia sepium

Copyright Andreas Miiller & Paul Westrich. This is an open access article distributed under the terms of the Creative Commons Attribution License
(CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

216 Andreas Miiller & Paul Westrich / Journal of Hymenoptera Research 95: 215-230 (2023)

Introduction

Nectar that is deeply hidden inside a flower is usually only accessible to flower-visiting in-
sects that possess long mouthparts. However, flower visitors with short mouthparts may
also obtain deeply concealed nectar either by i) biting a hole into the plant tissue near the
nectary through which the nectar is ingested (“primary nectar robbing”), ii) using an ex-
isting hole bitten by another flower visitor (“secondary nectar robbing”), iii) entering the
flower through the normal entrance without touching the sexual flower organs for exam-
ple due to small body size (“nectar thieving”), or iv) pushing the mouthparts between the
petals of a polypetalous flower from the side or base, thereby gaining abbreviated access
to the nectary (“base working”) (Inouye 1980; Irwin et al. 2010). Compared to nectar-
drinking insects that visit the flowers in a legitimate way, nectar robbers, nectar thieves
and side workers usually do not pollinate the flowers during the process of nectar uptake.

Bees behaving as primary nectar robbers are usually not equipped with specialised
morphological structures to get access to the nectar, but instead mostly use their un-
specialised mandibles to bite through the plant tissue (Macior 1966; Inouye 1983).
However, in the two bumblebee species Bombus (Alpigenobombus) wurflenii Radosz-
kowski and B. (Bombus) occidentalis Greene occurring in mountaineous areas of Eu-
rope and western North America, respectively, the apical edge of the female mandible
is not regularly rounded as in most other bumblebees but equipped with several teeth,
which are assumed to facilitate the perforation of the plant tissue (Inouye 1983; Reinig
and Rasmont 1988; Rasmont et al. 2021). As both species are regular and frequent
nectar robbers (Loken 1950; Maloof 2001; Goulson et al. 2013), the toothed man-
dibles likely represent a morphological specialisation for primary nectar robbing. The
two species are only distantly related (Cameron et al. 2007), suggesting that this spe-
cialisation has independently evolved twice in the genus Bombus. In contrast to most
other primary nectar robbers, carpenter bees of the genus Xylocopa do not use their
mandibles to perforate the flower tissue, but instead slit the flowers with their maxil-
lae (Schremmer 1972; Inouye 1983; Gerling et al. 1989). Interestingly, the maxillae
appear to be highly adapted to nectar robbing since the galeae are heavily sclerotised,
modified and tightly linked together forming a strong piercing organ, which is able to
perforate even rather hard plant tissue (Schremmer 1972).

The European mining bee species Andrena (Taeniandrena) lathyri Alfken (Andre-
nidae) exclusively collects pollen on plants belonging to the Fabaceae (Westrich and
Schmidt 1987). It is known to regularly act as a primary nectar robber on flowers of
Lathyrus and Vicia (Westrich 1989; Teppner et al. 2016), a behaviour that may be oblig-
atory in this species (Westrich 1989). Andrena lathyri differs from most other Andrena
species including the closely related representatives of the subgenus Taeniandrena by
a distinctly longer and strongly angled proboscis. This peculiar shape of the proboscis
has to the best of our knowledge never been noticed by bee taxonomists nor has its
function be explored by bee biologists. As both females and males of A. /athyri have
identical mouthparts and regularly rob flowers, the peculiar proboscis is hypothesised
here to be a morphological specialization for primary nectar robbing similar to the
toothed mandibles of some Bombus species and the piercing maxillae of Xylocopa.

Primary nectar robbing by Andrena lathyri 217

In the present study, we i) verify the narrow pollen host specialisation of Andrena
lathyri, ii) confirm the species’ habit as obligatory nectar robber, iii) analyse the mor-
phology of the specialised proboscis by comparing it with that of closely related
A, (Taeniandrena) species, iv) describe the bees’ behaviour during nectar robbing and
pollen collection, v) investigate the impact of primary nectar robbing on flower integ-
rity and fruit formation, and vi) discuss the hypothesis that the peculiar proboscis of
A, lathyri is a morphological adaptation to primary nectar robbing.

Methods

Bee species, flower species and study sites

Andrena lathyri is a 10-14 mm long ground-nesting solitary bee, which is distributed
over large parts of Europe and Turkey (Gusenleitner and Schwarz 2022). In Central
Europe, the species is widespread at lower elevations and regionally often abundant.
It belongs to the subgenus Taeniandrena, which comprises about 35 Palearctic species
with one species additionally introduced into the Nearctic (Gusenleitner and Schwarz
2002; Wood et al. 2021; Praz et al. 2022; Wood 2022; T. Wood, personal communica-
tion). All species of this subgenus, for which the pollen hosts are known, exhibit an
exclusive or very strong preference for the pollen of Fabaceae (Westrich 1989; Praz et
al. 2022). Together with A. aberrans Eversmann, which exclusively collects pollen on
few genera of the tribe Genisteae, such as Chamaecytisus (Westrich 2018), A. lathyri
is among the most strongly pollen-specialised representatives of A. (Zaeniandrena) ex-
ploiting only two closely related genera of the tribe Fabeae, i.e., Lathyrus and Vicia
(Westrich and Schmidt 1987; Schaefer et al. 2012). As the proboscis of A. lathyri is too
short to reach the nectaries at the base of the Lathyrus and Vicia flowers, nectar cannot
be ingested during legitimate flower visits. Instead, A. /athyri gains nectar by primary
nectar robbing on its pollen hosts.

Lathyrus pratensis L. and Vicia sepium L. have typical legume flowers with a five-part
calyx and corolla (Kugler 1970; Proctor et al. 2003; Fig. Ga—f). The calyx consists of
five sepals, which are fused over most of their length ending in five apical lobes (“calyx
teeth”). The corolla is composed of five petals, of which the uppermost petal is in dorsal
position (“standard”), the median two petals are in lateral position (“wings”) and the
lowermost two petals are in ventral position largely concealed by the lateral wings. ‘The
lowermost two petals are ventrally and apically fused forming a boat-shaped structure
(“keel”) that encloses the single pistil and ten stamina, of which the dorsalmost stamen
is free, whereas the filaments of the other nine stamina are fused to a staminal tube sur-
rounding the pistil. The flowers have a secondary pollen presentation in that the pollen
is shed at the late bud stage onto a dense brush of fine hairs near the apex of the style
(“stylar brush”), from where it is removed by pollen-collecting bees. Nectar is produced
at the base of the pistil in a nectary, which is longitudinally crossed by the filament of the
uppermost stamen, either slightly above the nectary’s upper rim in L. pratensis or slightly
below the upper rim in V. sepium (Fig. 6e, f) The nectar is accessible only from above

218 Andreas Miiller & Paul Westrich / Journal of Hymenoptera Research 95: 215-230 (2023)

the staminal tube, where there is a spacious empty flower interior due to the consider-
ably arched base of calyx and standard. To drink nectar from the flowers of L. pratensis,
flower visitors have to pass the proboscis through the rather narrow slit on either side of
the stamen filament, while the nectar is slightly more easily accessible in V. sepium due
to the lower position and the smaller width of the crossing stamen filament.

Field observations and experiments were performed in northern Switzerland near
Rekingen (47°33'59"N, 8°18'41"E; site 1), Dagmersellen (47°13'28"N, 7°58'49"E; site
2) and Wadenswil (47°13'06"N, 8°40'45"E; site 3) from May to June 2021 and 2022.

Pollen host preferences

To verify the pollen host specialization of Andrena lathyri to flowers of Lathyrus and
Vicia, we microscopically analysed the pollen contained in the hind leg scopa of 30
females collected at 30 different localities in Switzerland (n = 28) and Liechtenstein
(n = 2) between 1914 and 2019 using the method described by Westrich and Schmidt
(1986). Before removing pollen from the scopa, the amount of pollen was assigned to
five classes, ranging from 5/5 (full load) to 1/5 (filled to one-fifth). The pollen grains
were stripped from the scopa of one leg with a fine needle, embedded in glycerol gelatin
on a slide and identified at a magnification of 400x to family, subfamily or genus level.
While pollen of Lathyrus and Vicia can be easily distinguished from that of other Fa-
baceae taxa in Central Europe by light microscopy based on shape, ornamentation and
size of the hydrated pollen grains, there are no reliable characters to separate Lathyrus
from Vicia pollen in every case (Beug 2004). Therefore, pollen of these two closely
related taxa was recorded as Lathyrus/ Vicia type in the pollen samples examined.

Obligatory or facultative nectar robbing?

To clarify whether Andrena lathyri is an obligatory nectar robber on flowers of Lathyrus
and Vicia or whether it exploits numerous other plant taxa for nectar, we analysed the
flower-visiting data for A. /athyri contained in the database of the Wildbienen-Kataster
Baden-Wiirttemberg. At the time of data retrieval (August 2022), the database com-
prised 392 records of A. lathyri, which were distributed all over Baden-Wiirttemberg,
collected from 1988 to 2021 and provided by M. Haider, M. Klemm, V. Mauss, R.
Prosi, A. Schanowski and H.-R. Schwenninger. For 189 out of these 392 records, the
plant genus or plant species visited by A. /athyri was known.

Morphology of the proboscis

To analyse the morphology of the mouthparts of Andrena lathyri, the proboscis was ex-
amined under a stereomicroscope at a magnification of 40x and compared with that
of nine other western Palearctic Andrena species of the subgenus Taeniandrena, i.e.,
A. aberrans, A. afgeliella (Kirby), A. gelriae Van der Vecht, A. caesia Warncke, A. intermedia
Thomson, A. /eucopsis Warncke, A. poupillieri Dours, A. russula Lepeletier, and A. wilkella

(Kirby). For measurements, A. wilkella was selected as representative species for the

Primary nectar robbing by Andrena lathyri 219)

subgenus Taeniandrena as its proboscis was found to be morphologically identical to all
A. (Taeniandrena) species other than A. /athyri. For five females and five males each of
A. lathyri and A. wilkella originating from different localities in Switzerland, the length
of the glossa from the lowermost point of the basiglossal sclerite to its apex and the inter-
tegular width were measured with a micrometer scale to the nearest 0.025. For the same
individuals, the angle between the dorsal surface of the labium’s lateral sclerites and the
anterior surface of the glossa was determined on close-up photographs taken with a Nikon

D750 camera. The morphological terminology of the proboscis follows Michener (2000).

Nectar-robbing behaviour

To examine the behaviour of Andrena lathyri during primary nectar robbing, we ob-
served females and males gaining nectar from flowers of Lathyrus pratensis and Vicia
sepium with the aid of a threefold magnifying glass at site 1 on 16.6.2021 and at site 2
on 17.5.2022. In total, we observed about 90 nectar-robbing visits.

To understand the movements of the labium during nectar ingestion, two females of
Andrena lathyri were observed in the laboratory drinking sugar water from a small bowl.

To investigate whether the long and angled proboscis of Andrena lathyri fits into
the flower interior for nectar uptake, three consecutive sections of the labium of ten
females and ten males originating from different localities in Switzerland were meas-
ured with a micrometer scale to the nearest 0.025 mm under a stereomicroscope at 40x
magnification (Fig. 2a): i) length from the sclerotised base of the prementum to the
sclerotised base of the labial palpi (“basal section”), ii) dorsal length of the intermediate
section extending from above the sclerotised base of the labial palpi to and including
the basiglossal sclerite (“intermediate section”), and iii) length of the anterior surface
of the glossa from the dorsalmost point of the basiglossal sclerite to the tip of the
glossa (“apical section”). The measured lengths were averaged over all 20 individuals,
multiplied with ten and plotted onto a 10:1 drawing of an average-sized flower each
of Lathyrus pratensis and Vicia sepium to optically evaluate the labium’s fit to the flower
interior and its ability to reach the nectary.

Pollen-collecting behaviour

To examine the behaviour of Andrena lathyri during pollen collection, we observed
females harvesting pollen on flowers of Vicia sepium with the aid of a threefold mag-
nifying glass at site 2 on 17.5.2022 and at site 3 on 24.5.2022. In total, we observed
about 30 pollen-collecting visits.

Impact of nectar robbing on flower integrity and fruit development

To evaluate the damage caused to flowers by nectar-robbing individuals of Andrena
lathyri, we collected 50 robbed flowers each of Lathyrus pratensis and Vicia sepium at
site 1 on 4.5., 12.5. and 25.5.2022 and examined both corolla and calyx under a ster-
eomicroscope at a magnification of 20—40x.

220 Andreas Miiller & Paul Westrich / Journal of Hymenoptera Research 95: 215-230 (2023)

To investigate whether the fruits of flowers robbed by Andrena lathyri develop nor-
mally, 25 flowers of Vicia sepium that showed the typical sign of having been robbed,
ie., a longitudinal slit in the calyx (see below), were marked with coloured threads
at site 3 on 24.5.2022. Two weeks later, the development of the fruits of the marked
flowers was assessed.

Results

Pollen host preferences

All 30 female pollen loads of Andrena lathyri from Switzerland and Liechtenstein ex-
clusively consisted of pollen of the Lathyrus/ Vicia type (Fig. 1a). This finding supports
the species’ narrow pollen specialisation already postulated by Westrich and Schmidt
(1987), who found 46 pollen loads from Germany, Austria, Poland and Greece to be
composed only of Lathyrus and/or Vicia pollen.

Obligatory or facultative nectar robbing?

Based on the Wildbienen-Kataster dataset, 69 (87.3%) of 79 flower visits by females of
Andrena lathyri and 102 (92.7%) of 110 flower visits by males were recorded on flowers
of Lathyrus, such as L. niger (L.) Bernhardi, L. pratensis L. and L. vernus (L.) Bernhardi,
and of Vicia, such as V. angustifolia L., V. cracca L., V. sativa L., V. sepium L. and V. villosa
Roth (Fig. 1b). Considering that the females mostly collect both pollen and nectar dur-
ing the same flower visit (see below) and the males exploit the flowers only for nectar,
these figures suggest that A. /athyri only exceptionally gains nectar from flowers other
than Lathyrus and Vicia, rendering the species an almost obligatory nectar robber.

Morphology of the proboscis

The comparison of the proboscis of Andrena lathyri with that of nine other Andrena
species of the subgenus 7aeniandrena revealed that the morphological differences be-
tween A. /athyri and its relatives are restricted to the labium, whereas the construction
of the maxillae is identical.

The labium of Andrena (Taeniandrena) species consists of five main sclerotised parts
(Fig. 2a, b): i) the prementum, which extends till the base of the labial palpi, ii) the two
four-segmented labial palpi, which attach laterally to the end of the prementum, iii)
a pair of lateral sclerites, which cover the base of the paraglossae from above and form
together with the adjacent basiglossal sclerite an intermediate section between the end
of the prementum and the anterior surface of the glossa, iv) the two paraglossae, which
attach distal to the ventral end of the prementum, run along the lower margin of the
two lateral sclerites and are distally bent outwards, and v) the glossa, which originates
between the base of the paraglossae, tapers towards its apex and is densely beset with
annulate hairs below the basiglossal sclerite.

Primary nectar robbing by Andrena lathyri 221

100 100
90 90
80 80
70 70
60 60
50 50
40 40
30 30
20 20
10 (a) 10 (b)
0 0
pollen loads (n = 30) females (n= 79) males (n= 110)

Figure |. Pollen and nectar sources of Andrena lathyri a composition of female pollen loads b flower

visits. Dark grey = Lathyrus and Vicia species, light grey = other plant species.

The labium of Andrena lathyri differs in two main respects from that of all other
A. (Taeniandrena) species represented here by A. wilkella (Fig. 2). First, the glossa is on
average 1.57 mm long in A. /athyri (range = 1.48—1.68 mm, n = 5 females and 5 males),
whereas it is on average 0.50 mm long in A. wilkella (range = 0.45—0.55 mm, n = 5 fe-
males and 5 males). By correcting for the difference in body size, which is 10% smaller in
A. wilkella based on intertegular width, the glossa of A. /athyri is about 2.9x longer than
that of A. wilkella. Second, the angle between the dorsal surface of the lateral sclerites and
the anterior surface of the glossa is on average 100.8° in A. /athyri (range = 91-109°, n=5
females and 5 males), whereas it is on average 140.9° in A. wilkella (range = 130-152°,
n= 5 females and 5 males). These differences result in a much longer and distinctly more
strongly angled labium in A. /athyri compared to most other Andrena species including
the closely related representatives of the subgenus Taeniandrena.

Nectar-robbing behaviour

The behaviour of nectar-robbing individuals of Andrena lathyri, which was very uni-
form, identical in both sexes and invariant between flowers of Lathyrus pratensis and Vicia
sepium, can be divided into three phases. In the first phase, the bees crawled headfirst to
the side of the flower and inserted the half-extended proboscis and the lower half of the
head under the lower margin of the standard (Figs 3a, 6c, d). In the second phase, the
bees moved the head between the inner side of the standard and the outer side of the
wing towards the base of the flower (Fig. 3b); when the advancing head reached the calyx,
the outer mandible was spread out (Fig. 3c), and by moving the head further towards the
flower base, the calyx was torn open between the lowest and the second lowest calyx tooth
(Fig. 3d); during advancing, the outer mandible was repeatedly moved up and down,
which probably facilitated the process of tearing the rather hard calyx tissue by acting

222 Andreas Miiller & Paul Westrich / Journal of Hymenoptera Research 95: 215-230 (2023)

Figure 2. Mouthparts of Andrena lathyri and Andrena wilkella in \ateral view a labium of A. lathyri

b labium of A. wilkella ¢ maxilla and labium of A. /athyri. pm = prementum, Ip = labial palpus, Is = lateral
sclerite, bs = basiglossal sclerite, pg = paraglossa, gl = glossa; I = basal section, II = intermediate section,

HI = apical section.

as an abutment or support, but not as cutting tool; instead, the calyx tissue tore due to
the pressure of the advancing head; sometimes, a soft crackling sound was heard, which
probably occurred when the tissue tore; at the end of this phase, the outer mandible
rested on the calyx and the inner mandible, the proboscis and one compound eye were

completely hidden inside the flower (Fig. 3d). In the third phase, the bees — holding

Primary nectar robbing by Andrena lathyri 225

Figure 3. Nectar-robbing behaviour of Andrena lathyri. See text for details.

224 Andreas Miiller & Paul Westrich / Journal of Hymenoptera Research 95: 215-230 (2023)

their head obliquely upwards from its ventral position relative to the flower —extended
the proboscis inside the flower in longitudinal direction to the head upwards, so that the
uppermost point of the angled proboscis was close to the upper roof of the flower base
and the lowermost point, i.e., the apex of the glossa, reached the nectary from above to
drink nectar (Fig. 4); after nectar uptake, the bees withdrew the proboscis from the calyx
slit and left the flower (Fig. 3e).

In all observed nectar-robbing visits (n = 90), only one side of the calyx was slit,
suggesting that the proboscis can empty the nectary from one side despite the longitu-
dinally crossing stamen filament and that calyces slit on both sides originate from two
different visits by Andrena lathyri.

How often individuals of Andrena lathyri gained nectar by secondary nectar rob-
bing, i.e., by using an already existing calyx slit, could not be quantified exactly, be-
cause the observer’s view of the flower base became obscured as soon as the bee crawled
to the side of the flower to rob nectar. However, secondary nectar robbing occasionally
occurred (Fig. 3f).

The observation of nectar-drinking females of Andrena lathyri in the laboratory
revealed that the glossa can be moved far forwards and backwards in longitudinal di-
rection to the body due to a ventral joint adjacent to the distal end of the prementum.
In addition, the haired part of the glossa itself is movable to all sides, allowing for its
precise guidance within the flower. In repose, the glossa is folded back over the dorsal
surface of the prementum, which is embedded between the stipites of the maxillae
within the proboscidial fossa on the underside of the head. For nectar ingestion, the
prementum is moved downwards and forwards followed by the folding out of the
glossa. At maximum extension of the proboscis, the sclerotised base of the prementum
is situated roughly underneath the labrum resulting in a long maximal reach of the
proboscis. By moving the prementum forward at different distances and/or bending
the joint at the end of the prementum at varying angles, the mouthparts have a consid-
erable flexibility to adjust to the specific interior of the host flowers, which is expected
to slightly differ among the different species of Lathyrus and Vicia that are exploited by
A, lathyri (see above).

The basal, intermediate and apical section of the labium of Andrena lathyri meas-
ured on average 1.91 mm, 0.30 mm and 1.74 mm, respectively (n = 10 females and
males each). The true-to-scale plotting of these three sections, i.e. the labium at its
maximum extension, onto flower drawings of Lathyrus pratensis and Vicia sepium re-
vealed - in combination with the observed position of the bee’s head during primary
nectar robbing and the fact that the nectaries can be accessed only from above (see
above) - that the bee’s mouthparts precisely correspond to the dimensions of the flower
interior and the position of the nectary (Fig. 4a—c).

Pollen-collecting behaviour

To collect pollen on flowers of Vicia sepium, the females of Andrena lathyri pushed the
standard upwards with the front of the head, which created the necessary space for the
movements of the fore legs (see below), and simultaneously pressed the wings sideways

Primary nectar robbing by Andrena lathyri 225

Figure 4. Maximally extended labium of Andrena lathyri plotted true to scale into the flower interior

a Lathyrus pratensis b, € Vicia sepium. Black = labium, grey = nectary and calyx slit.

down with the middle and hind legs, which caused the keel to move downwards and
the pollen-bearing stylar brush to emerge from the tip of the keel (Fig. 5a). The bees
then harvested the pollen from the stylar brush by rapid strokes of the fore legs, which
resulted in pollen masses adhering to both the pilosity of the foretarsi and the long
hairs covering the underside of the maxillar stipites (Fig. 5b).

During most flower visits on Lathyrus pratensis and Vicia sepium, the females of
Andrena lathyri first collected pollen, before crawling to one side of the flower to rob
nectar. Occasionally, however, pollen-only and nectar-only visits also occurred.

Impact of nectar robbing on flower integrity and fruit development

Of the 50 corollae each of Lathyrus pratensis and Vicia sepium examined for damage
by nectar-robbing individuals of Andrena lathyri, 81 (81%) were intact and 16 (16%)
could not be properly assessed as the corolla was partly withered due to the late devel-
opment stage of the flower; only three (3%) flowers of V. sepium were found to have
a 1.8-5.8 mm long longitudinal crack near the lower margin of one of the two lateral
wings, most probably caused by A. /athyri during nectar robbing.

Primary nectar robbing by Andrena lathyri on Lathyrus pratensis and Vicia sepium
invariably resulted in a longitudinal slit in the calyx (Fig. 6a, b). This slit was located
between the lowest and the second lowest calyx tooth in all 100 flowers examined with
the exception of one flower of V. sepium, where the slit was located between the second
lowest and the third lowest calyx tooth; interestingly, the corolla of the latter flower
was one of the very few damaged by nectar robbing (see above), suggesting that this
flower might possibly have been robbed by an unexperienced forager. The calyx was
slit only on one side in 89 (89%) flowers examined, whereas it was slit on both sides in
11 (11%) flowers, probably resulting from two different flower visits (see above). The
length of the calyx slit was significantly shorter in L. pratensis (range = 0.7—2.1 mm,
median = 1.3 mm, n = 54) than in V. sepium (range = 2.0-3.9 mm, median = 2.8 mm,
n = 57) (Mann-Whitney-U-test, U = 2.5, p < 0.001). In contrast, the distance between
the end of the slit and the base of the flower, where the nectary is located in both

226 Andreas Miiller & Paul Westrich / Journal of Hymenoptera Research 95: 215-230 (2023)

Figure 5. Pollen-collecting behaviour of Andrena lathyri. See text for details.

species, did not significantly differ between L. pratensis (range = 1.4—2.6 mm, median
2.1 mm, n = 54) and V. sepium (range = 1.4—2.9 mm, median = 2.2, n = 57) (Mann-
Whitney-U-test, U = 1325.5, p = 0.206).

In 21 (84%) of the 25 marked flowers of Vicia sepium, the fruits developed nor-
mally in spite of having been robbed by Andrena lathyri (Fig. 6g, h), in one flower (4%)
the ovary was withered and three flowers (12%) dropped off the plant for unknown
reasons, overall suggesting a negligible negative impact of primary nectar robbing on
fruit development.

Discussion

The present study verifies the narrow pollen specialization of Andrena lathyri to flowers
of Lathyrus and Vicia (Fabaceae) and confirms the species’ habit as a largely obligatory
nectar robber on its pollen hosts.

Compared to the great majority of Andrena bees including the most closely related
species (see Danforth et al. 2019), the proboscis of A. lathyri is exceptionally long
due to a considerable elongation of the glossa and strongly bent due to an almost
right angle between the short intermediate section of the labium and the glossa. ‘This
specialised proboscis precisely fits into the flower interior of Lathyrus and Vicia and
its long and angled shape enables the bees to reach the nectary from above, which is
the only way to gain nectar given the ventral position of the bee’s head during nectar
uptake. Because of this exact match between bee proboscis and host-flower interior, the
specialised mouthparts of A. /athyri are interpreted here as a morphological specialisa-
tion for primary nectar robbing, which is in line with the findings that the proboscis is
not actively involved in pollen harvesting and that females and males, which both rob
flowers, possess identical mouthparts.

Primary nectar robbing by Andrena lathyri 227

Figure 6. Flowers of Vicia sepium (left) and Lathyrus pratensis (right) a, b longitudinal slit in calyx
due to primary nectar robbing by Andrena lathyri c, d flower with calyx removed showing lower side of
standard (arrow), under which A. /athyri moves head and proboscis towards flower base e, f flower in top
view with calyx, standard and wings removed showing nectary (ne), which is accessible only from above

g, h normally developing fruits despite slit in the calyx (arrow) caused by A. lathyri.

Beside this morphological adaptation to primary nectar robbing, Andrena lathyri
also exhibits behavioural specialisations, which include the insertion of proboscis and
head under the lower margin of the standard of the Lathyrus and Vicia flowers and

228 Andreas Miiller & Paul Westrich / Journal of Hymenoptera Research 95: 215-230 (2023)

the subsequent slitting of the calyx to a depth necessary to reach the nectary with
the specialised proboscis. Interestingly, by slitting the calyx over a shorter distance
in L. pratensis than in V. sepium, which has a longer calyx, nectar-robbing individu-
als of A. lathyri keep in both flower species the same distance between the end of the
slit and the flower base, where the nectary is located. Slitting the calyx of L. pratensis
over a similar length as in V. sepium would render nectar uptake by the specialised
mouthparts impossible as in this case the proboscis would be too close to the flower
base to reach the nectary from above. ‘This finding reveals an amazing ability of nectar-
robbing individuals of A. /athyri to adapt their behaviour to the different calyx lengths
of their hosts.

The flowers of Lathyrus and Vicia species are characterised by a strongly arched
base of calyx and standard leading to an almost right angle between the posterior side
and the upper side of the flower base. This characteristic, which also occurs in other
genera of the tribe Fabeae, such as Pisum or Vavivolia, results in a spacious flower in-
terior above the nectary providing enough space for the movements of the specialised
proboscis of Andrena lathyri. In contrast, the flower base of many other Fabaceae gen-
era, such as Genista, Hippocrepis, Lotus, Melilotus, Onobrychis or Trifolium, is usually
distinctly less strongly arched hardly leaving enough space for A. /athyri to gain nectar
with its mouthparts. This difference in the shape of the flower base might contribute
to the narrow flower specialisation of A. /athyri and probably renders host shifts to
Fabaceae species other than those of the Fabeae difficult.

Primary nectar robbing by Andrena lathyri does not damage any flower parts ex-
cept for a short slit on one side of the calyx, which is consistent with the findings that
fruit development was not substantially impaired by nectar robbing and that at sites
where A. /athyri was common many calyces at the base of well-developed fruits of both
Lathyrus pratensis and Vicia sepium were slit. In contrast to the males of A. lathyri,
which never come into contact with the sexual flower organs due to their illegitimate
nectar visits, females are expected to pollinate the flowers during pollen collection,
since both Lathyrus and Vicia have homogamous flowers with female and male re-
productive organs maturing at the same time (Kugler 1970). Thus, primary nectar
robbing by A. /athyri does not seem to have any significant negative effect on the re-
production of the bee’s exclusive host plants, even more so as the flowers of L. pratensis
and V. sepium are visited and pollinated by a multitude of different long-tongued bee
species (Westrich 1989).

Conclusions

The peculiar proboscis of Andrena lathyri is one of the few known examples of a mor-
phological adaptation to primary nectar robbing in bees and tightly binds the bee
to its specific host plants, whose fruit development is not negatively affected by the
illegitimate nectar gain.

Primary nectar robbing by Andrena lathyri 229)

Acknowledgements

The Wildbienenkataster Baden-Wiirttemberg and Rainer Prosi generously provided
an excerpt of all data of Andrena lathyri from Baden-Wiirttemberg containing nu-
merous flower-visiting records. Philipp Heller, Christophe Praz and André Rey gave
hints to localities, where A. /athyri occurred in good numbers. Urs Weibel helped with
fieldwork. Thomas Wood provided information on the morphology of the proboscis
of A. (Taeniandrena) species. Michael Greef (ETH Zurich), Jessica Litman (Muséum
d’Histoire Naturelle de Neuchatel) and Christophe Praz loaned females of A. lathyri
for pollen removal and analysis. Mark Winston helped with the morphological inter-
pretation of the proboscis of A. /athyri. Anne Greet Bittermann from ScopeM (ETH
Zurich) took the SEM image. Comments by J. Cane, J. Neff, E. Scheuchl and T. Wood

improved the manuscript.

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