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Gender Specific Brood Cells in the Solitary Bee Colletes halophilus (Hymenoptera; Colletidae)
Eveline F. Rooijakkers
Marinus J. Sommeijer
We studied the reproductive behaviour of the solitary bee Colletes halophilus based on the variation in cell size, larval food amount and larval sex in relation to the sexual size dimorphism in this bee. Brood cells with female larvae are larger and contain more larval food than cells with males. Occasionally males are reared in female-sized cells. We conclude that a female C. halophilus in principal anticipates the sex of her offspring at the moment brood cell construction is started. Additionally a female is able to 'change her mind' about the sex of her offspring during a single brood cell cycle. We present a model that can predict the sex of the larvae in an early stage of development.
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Colletes halophilus Verhoeff (Hymenoptera; Colletidae) is a short-tongued mining
bee, which may nest in large aggregations (Westrich 1989). This bee belongs to the
Colletes succinctus group, among others with C. succinctus and C. hederae. The
taxonomic and evolutionary position of this and related species is still subject of
study (Kuhlmann et al. 2007).
Colletes halophilus occurs in tidal areas with brackish water. The flight season in
The Netherlands is from August up to mid October (Peeters et al. 1999). Aster
tripolium and Sonchus arvensis are reported to be major food sources of C.
halophilus (Westrich 1989). However, we recently found that the larval food nearly
exclusively contains A. tripolium pollen (Sommeijer et al. 2009). All colletid species
are solitary and most are soil-nesting (Michener 2000). According to OToole and
Raw (1999) the nest of C. halophilus consists of several brood cells, with five or
six cells in a completed nest. Brood cells are constructed following the strict
sequence of: excavation, lining of cell wall, provisioning, oviposition and closure
(Michener 1964).
A remarkable characteristic shared by all colletid bees is the material used to line
the brood cells (Michener 2000). The female spreads the lining material, mostly
Dufours gland secretions consisting of polyesters (specifically laminesters), with the
glossa onto the inner wall. Probably, added salivary-gland secretions cause
polymerisation and solidification (Michener 2000). This results in a tough inner
wall of the cell that is translucent and cellophane-like (Michener 1964).
Another feature is that, unlike most other solitary bees, the provisions of most
colletid bees are liquid. This is a suspension of pollen in a liquid that consists
probably of floral nectar and glandular secretion (Michener 2000; OToole and Raw
1999; Sommeijer et al. 2009). Instead of laying the egg directly on the food mass,
Colletes females place the egg on the upper part of the cell wall where it hangs
above the surface of the liquid food (Michener 2000).
In 1943 Verhoeff described C. halophilus as a separate species and reported that
males are 1112 mm long and females 1214 mm long. While doing research on this
bees foraging behaviour (Sommeijer et al. 2009) we noted a remarkable size
variation in the brood cells and in the amount of larval food within. Since female C.
halophilus are larger then males (Verhoeff 1943) we hypothesized that females are
reared in larger cells with a greater amount of larval food. Our goal was to confirm
this hypothesis and construct a model that can accurately predict the sex of the
offspring using very simple parameters.
Bees and Site
Colletes halophilus occurs in a scattered pattern along the North Sea and Atlantic
coastline from the Netherlands (Peeters et al. 1999) southwards to Brittany and
Aquitaine (Genoud and Dittlo 2007; Vereecken pers. comm. 2007). It is further
reported to occur in the southwestern region of the UK (Scott Roberts pers. comm.
2006). In the Netherlands C. halophilus occurs along the coast, particularly in and
near tidal wetlands in the Scheldt delta and on some Friesian islands (Peeters et al.
1999). The present study was performed with material collected from the
aggregation site in the nature reserve Het Verdronken Land van Saeftinghe
(3,000 Ha). This aggregation of about 3 Ha is one of the largest aggregation sites of
C. halophilus in the Netherlands.
Analytic Procedures
A total of 90 brood cells were collected shortly after the flight season in 2005 (on
711-2005). To determine the size of the brood cells, height (H) and diameter (D) were
measured 2 days later (accuracy of 0.5 mm). Until that time cells were kept in sand.
Figure 1 illustrates how measurements were taken.
Fig. 1 Illustration of Colletes
halophilus brood cells, indicated
are how height and diameter
measurements were taken.
To assess the amount of larval food in the brood cells, the weight (W) of the cells
was measured with an accuracy of 0.1 mg (on 11-11-2005). The cells were weighed
with larvae inside. Body weights of the still very small larvae were assumed to
compensate for most of the food weight loss due to feeding. Weight loss due to
respiration and growth were for the releva (...truncated)