Original article Why males of leafcutter bees hold the females’ antennae with their front legs during mating D Wittmann B Blochtein 1 Laboratório de Pesquisas Biológicas da Universidade de Tübingen, PUC/RS, Av Ipiranga 6681, 90620 Pôrto Alegre, Brazil; 2 Zoologisches Institut, LS Entwicklungsphysiologie, Universität Tübingen, Auf der Morgenstelle 28, 72076 Tübingen, Germany (Received 12 September 1994; accepted 21 December 1994) Summary — Epidermal odor glands were found in the front leg basitarsi of males of various subgenera of leafcutter bees from the Old and New World. In these males, the legs, the mandibles and the head capsules are modified. During mating these structures are used by the male to grasp the female and bring the antennae of the female in close contact with the openings of the basitarsal odor glands of the front legs. Odor glands and analogous modifications of associated body structures were also found in xylocopine bees and in a sphecid wasp. Megachilidae / Xylocopini / Sphecidae / odor gland / mating INTRODUCTION of orchid bees (Euglossini) are the only example where structurally modified legs serve to collect materials. With dense tufts A conspicuous sexual dimorphism in legs of branched hairs on their front is common in all taxa of bees in which legs (Moure, these males scrape from females have corbiculae or scopae on their 1950), fragrances and transfer them hind legs to transport pollen. In floral oil col- flowers (mainly orchids) lecting bees (Anthophoridae: Centridini, Exo- into pouches of their inflated hind legs malopsini, Tetrapediini; Ctenoplectridae: (Dressler, 1982). The front and/or middle Ctenoplectra Smith 1857; Melittidae: legs of males of some oil-collecting bees, Macropis Klug 1809, Rediviva Friese 1911), namely those of Monoeca Lepeletier 1828 the front and/or middle legs bear scraping and Paratetrapedia Moure 1941 bear combs and oil-collecting structures (Vogel, 1966, and scrapers which closely resemble the 1974; Neff and Simpson, 1981). However, oil-collecting structures of the females. The modified legs are also found in males. Males function of these modifications is unknown. Well known are the adhesive and clasper The purpose of this study was to eluci- organs on the hind legs of Apis drones date the function of some structural modifi- which serve to hold the female during cop- cations in front legs and other body parts ula in flight (Ruttner, 1975; Koeniger and of megachilid bees and to compare them Koeniger, 1991). with similar structures in carpenter bees and The function of the dilated front legs of sphecid wasps. Preliminary reports on our have been elsewhere males of some species of carpenter bees findings published Blochtein and (eg, Xylocopa (Platynopoda) tenuiscapa (Wittmann et al, 1992; Westwood 1840, X (Platynopoda) magnifica Wittmann, 1993). (Cockerell 1929), X (Platynopoda) latipes (Drury 1773), X (Platynopoda) perforator MATERIALS AND METHODS Smith 1861) is only partially understood. For carpenter bees, Anzenberger (1978) and Osten (1989) found the following func- This study was initiated with a survey of modi- tion of the modified legs during copula in fied body parts in males of megachilid bees from the Old and New World. As a flight: males grasp the females in flight and working hypothesis, a general pattern of the function of these struc- place their front or middle legs, which have tures during mating was developed and predic- of over the fringes long setae, compound tions were made on the presence of odor glands eyes of their mate. Very little is known about in the modified legs of megachilid and xylocopine the function of the modified legs in some bees and a sphecid wasp. species of Nomada Scopoli 1770 and in var- The external morphology of preserved males ious genera and subgenera of Megachile of about 150 New and Old World megachilid (eg, Eumegachile Friese 1899, Pseudo- species, 2 xylocopine bee species from India and centron Mitchell 1934, Sayapis Titus 1905, Africa (X (Platynopoda) tenuiscapa, X torrida Westwood 1838 and 3 Melanosarus Mitchell 1934, Acentron (Mesotrichia) species from Brazil (X (Megaxylocopa) frontalis Mitchell 1934) Chalicodoma Lepeletier 1841 (Oliver 1789), X (Dasyxylocopa) bimaculata and Fidelia Friese 1899. Mitchell (1930, Friese 1903, X (Neoxylocopa) nigrocincta Smith 1980), Batra (1978) and Osten (1989) 1854) and 1 European sphecid wasp species assume that the modified front legs in (Crabro cribrarius (Linnaeus, 1758)) was studied Megachile Latreille 1802 play a role during under a stereomicroscope at 40-60x. Dry were in mating. The few ethological observations museum specimens placed boiling water for 10 min to make them flexible during spreading on of bees with modified copula megachilid of the legs and thus facilitate access to the other- front legs (Batra, 1978) indicate that the wise hidden structures. males also cover the eyes of the compound In search of glandular structures, we studied with setae at female fringes of located their the inner morphology of tarsal segments in some front leg tarsi. megachilid bees (M ericetorum Lepeletier 1841, Modifications that resemble those found M willughbiella (Kirby 1802), M rotundata (Fabri- cius 1784), M (Acentrina) anthidioides in and males are also megachilid xylocopine Radoszkowski 1874, M lagopoda (Linnaeus known from As a sphecid wasps. conspicu- 1761), M (Acentron) spp, M (Pseudocentron) ous feature, the front leg tibia of many males spp). Furthermore, we searched for glandular is widely dilated, forming a concave shield. structures in the front legs of X (Platynopoda) During mating, the male places these shields perforator and X (Mesotrichia) torrida, and the C cribrarius Linnaeus. For the scan- over the compound eyes of the female sphecid wasp electron studies et al, 1979; Low and Wcislo, ning microscopic (SEM), speci- (Matthews mens were mounted on aluminium stubs and 1992). Little attention has been paid to the coated with gold and palladium. To study the inte- use of the front leg tarsal segments, the rior surface of the tarsal segments, they were cut mandibles and the head capsule in mating. open, macerated in 5% KOH for 12-24 h, de- hydrated in 70, 80, 90 and 100% acetone, and By doing so, the concave fan of long setae dried at 30°C. covers the compound eyes of the female. To test our hypothesis on the use of modified With his middle legs, he presses the body structures during mating, the behavior in M female’s wings downward and thus prevents was studied at the Botanical Gar- willughbiella her from taking off. The hind legs are posi- den of the of from June to University Tübingen tioned under the female’s abdomen and August 1992 and 1993. We also analysed the A closer look at the copulation behavior of laboratory reared, newly pulled slightly upwards. emerged M rotundata using video recordings. morphology of these body parts gives us a These matings occurred in glass boxes measur- better understanding of their positioning and ing 7 x 2 x 2 cm. function during copulation (fig 1). Males of megachilid species (M (Acentrina) apicipennis Schrottky 1902, M (Pseudocentron) Modified structures of the mandibles terrestris Schrottky 1902) collected in Brazil, and and the head of M the European M willughbiella, captured on flowers capsule willughbiella in Germany, had their heads and front legs removed, washed and extracted in pentane. Gas Below their articulation, the mandibles pro- chromatographic and mass spectroscopic anal- trude into a ’basal’ or ’inferior process’ yses were carried out at the Institute of Organic according to Moure (1943) and Mitchell Chemistry and Biochemistry, University Ham- (1980). These structures have their coun- burg, on a Varian MAT 311. A coupling system in the head using a 50 m x 0.25 mm (id) fused silica column terpart capsule immediately beneath the articulation each mandible. with a FFAP-CB as a stationary phase was of employed. That is, the head capsule is deeply exca- vated and projects ventrally into a pointed protuberance (fig 2). RESULTS Function: When the male has mounted the female, he pushes her antennae into the wide buccal the Although our focus in this paper is on the space by grasping scapi functional morphology of modified body with the mandibular basal processes. By the the male parts, a short description of the positioning widely opening mandibles, of the male during copulation in M willugh- presses the flagellum of each antennae into the excavations of the head In this biella, is also presented. Subsequently, we capsule. one can see the female’s compare it with the positioning of males in M position, flagella out on either side the rotundata. A more detailed description of sticking through gap the mating and territorial behavior in M between the male’s mandibles and the head willughbiella has previously been reported by capsule. Blochtein and Wittmann (1993). Modified structures of the male’s front leg coxa of M willughbiella Case study: overview of the copulation behavior in M willughbiella The coxae of the front legs protrude into long spines which are curved anteriorly. At In M willughbiella, the male mounts the their tip and their margins, these coxal have of female in such a position that his head is spines patches sensory receptors above and in front of the female’s head. He (fig 3). then places his dilated front leg basitarsi Function: When the male has mounted the over the flagella of the female’s antennae female, he places his coxal spines behind and presses them downwards to the fronts. the head capsule of the female. This not only allows the male to tightly hold the head cover a very small number of ommatidia of of the female, but also forces the head the compound eyes of the female. slightly downwards and thus prevents any roll and movements. pitch Modifications of the middle and the hind legs of M willughbiella Modified structures of the front leg tarsus of M willughbiella The tibiae of the middle and the hind leg are slightly inflated and curved ventrally.