Exocrine Glands in the Legs of the Social Wasp Vespula Vulgaris Catherine Nijs & Johan Billen
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Lirias 1 Submitted to Arthropod Structure and Development 2 3 4 5 Exocrine glands in the legs of the social wasp Vespula vulgaris 6 7 8 Catherine Nijs & Johan Billen 9 Zoological Institute, University of Leuven, Naamsestraat 59, box 2466, B-3000 10 Leuven (Belgium) 11 Keywords: morphology, legs, exocrine glands, social wasps 12 Running title: Exocrine glands in the legs of Vespula vulgaris 13 Corresponding author: 14 Johan Billen, University of Leuven, Zoological Institute, Naamsestraat 59, box 2466, 15 B-3000 Leuven, Belgium 16 Tel: (32) 16 323975 17 Fax: (32) 16 324575 18 E-mail: [email protected] 19 1 20 Abstract 21 This study brings a survey of the exocrine glands in the legs of Vespula 22 vulgaris wasps. We studied workers, males, virgin queens as well as mated queens. 23 A variety of 17 glands is found in the different leg segments. Among these, five 24 glands are novel exocrine structures for social insects (trochanter-femur gland, 25 ventrodistal tibial gland, distal tibial sac gland, ventral tibial gland, and ventral 26 tarsomere gland). Most leg glands are present in the three leg pairs of all castes. 27 This may indicate a mechanical function. This is likely for the numerous glands that 28 occur near the articulation between the various leg segments, where lubricant 29 production may be expected. Other possible functions include antenna cleaning, 30 acting as a hydraulic system, or pheromonal. Further research including leg-related 31 behavioural observations and chemical analyses may help to clarify the functions of 32 these glandular structures in the legs. 33 2 34 1. Introduction 35 Apart from some specific functions such as predation (e.g. mantis and 36 predatory bugs), digging (e.g. mole crickets) or pollen collection (e.g. bees and 37 bumblebees), the legs form the major means of locomotion that insects use in 38 addition to their wings. Although this may involve special developments such as 39 rowing devices in water beetles or massive femora for jumping in grasshoppers, most 40 insects have slender legs for walking. It is obvious that the legs contain muscle and 41 nerve fibres to control the necessary movements. It may be less evident to find also 42 other tissues inside the legs, although a recent review revealed that ants can harbor 43 up to 20 different exocrine glands in their legs (Billen, 2009). It was already known 44 that also bees can have a dozen glands in their legs (Cruz Landim et al., 1998; Cruz- 45 Landim, 2002), while a follow-up study in workers of just a single stingless bee 46 species Frieseomelitta varia revealed 15 different glands in their legs (Billen and 47 Vander Plancken, 2014). The only known reports on the occurrence of glands in 48 wasp legs are the description of the arolium glands in Polistes annularis workers and 49 queens (Billen, 1986) and a paper by Beani and Calloni (1991) who found scattered 50 gland cells in the tarsomeres, tibia and femur of males and females of Polistes 51 dominulus. This overwhelming variety of glands in the legs of ants and bees inspired 52 us to examine and compare the situation in the legs of wasps as the third major 53 Hymenopteran group of social insects. 54 2. Material and methods 55 Wasps of the three castes (3 individuals each of workers, virgin and mated 56 queens, and males) were obtained from nests that were collected in Leuven, 57 Belgium. In order to allow proper penetration of the various solutions during tissue 58 preparation, we made transverse cuts of each leg in the femur, tibia and basitarsus. 59 This resulted in smaller tissue pieces formed by coxa-trochanter-femur, femur-tibia, 60 tibia-basitarsus, and the distal tarsomeres. The leg fragments were fixed in 2% 61 glutaraldehyde (buffered at pH 7.3 with 50 mM Na-cacodylate and 150 mM 62 saccharose), followed by postfixation in 2% osmium tetroxide in the same buffer. This 63 was followed by dehydration in a graded acetone series and embedding in araldite. 3 64 Serial semithin sections of 1 µm were made with a Leica EM UC6 ultramicrotome 65 using a diamond knife, stained with methylene blue and thionin, and examined with 66 an Olympus BX-51 microscope. Thin sections for electron microscopy with a 67 thickness of 70 nm were double stained with lead citrate and uranyl acetate, and 68 viewed in a Zeiss EM900 microscope. For estimation of cell numbers of class-3 69 glands, we first calculated the average diameter of the secretory cells for a particular 70 gland by looking at the biggest cells appearing on sections. The cumulative number 71 of cells appearing on serial sections at an interval equivalent to this diameter was 72 then considered as the total number of secretory cells of this particular gland. In 73 addition to the 3 individuals of each caste that were prepared for sectioning, we also 74 mounted the legs of 3 other individuals on stubs, that were gold coated and 75 examined with a JEOL JSM-6360 scanning microscope. 76 3. Results and discussion 77 We found a total of 17 exocrine glands in the legs of Vespula vulgaris, that we 78 present here in the order from most proximal (coxa) to most distal (pretarsus). In our 79 descriptions, we follow the standard classification as given in the pioneer paper of 80 Noirot and Quennedey (1974), that distinguishes between epithelial class-1 glands 81 and class-3 glands, that are formed by a number of bicellular units. Both class-1 and 82 class-3 glands can open through the external tegumental surface, or they can open 83 into a internal reservoir, where secretion can be temporarily stored (Fig. 1). For cell 84 measurements in queens, virgin and mated queens are pooled together, as no clear 85 differences were found between both reproductive conditions. All longitudinal 86 sections are shown with the distal side to the right. A general survey of all glands is 87 given in Figure 2 and Table 1. 88 3.1. Coxal gland 89 In their distal part, near the articulation with the trochanter, the coxae of the 90 three leg pairs in all castes contain clusters of spherical class-3 secretory cells (Figs 91 3A,B). Queens have 50-70 cells with a diameter of 43.9 ± 4.2 µm, workers have 92 approx. 30 cells with a diameter of 33.6 ± 1.0 µm, while males have 10-20 cells with 93 a diameter of 25.1 ± 4.1 µm. The cells have centrally located round nuclei with a 4 94 diameter of approx. 10 µm, and occur both at the dorsal and ventral side. Their ducts 95 open through the dorsal and ventral cuticle, as well as through the articulation 96 membrane between coxa and trochanter. The location at this articulation site in the 97 three castes may be indicative for a lubricant function, although the clearly bigger 98 gland in queens, as expressed in both a considerably higher cell size and cell 99 number, may also be linked with a pheromonal function. Ultrastructural examination 100 reveals the occurrence of a well developed smooth endoplasmic reticulum (Fig. 3D), 101 that can be linked with the elaboration of non-proteinaceous lubricant substances. 102 Similar class-3 secretory cells have also been found in the three leg pairs of 103 Pachycondyla obscuricornis ants, where they occur both in the proximal region of the 104 coxa where it articulates with the thorax, as in the distal region near the junction with 105 the trochanter (Schoeters and Billen, 1993). Cruz Landim et al. (1998) described 106 clusters of class-3 secretory cells in the proximal part of the coxae of several 107 meliponine bee species, although the opening site of their ducts could not be traced. 108 Distally occurring coxal glands, with ducts opening through the intersegmental 109 membrane with the trochanter and a presumed lubricant function, as here described 110 for Vespula vulgaris, were also reported in Frieseomelitta varia stingless bees (Billen 111 and Vander Plancken, 2014). Class-3 coxal glands have also been found in neotenic 112 Prorhinotermes simplex termites (Šobotník et al., 2003). 113 3.2. Trochanter-femur gland 114 The trochanter of the three leg pairs in all castes contains in its distal region a 115 cluster of 10-15 round class-3 secretory cells, their accompanying ducts open 116 through the articulation membrane with the femur (Fig. 3C). They have a diameter of 117 35.6 ± 9.5 µm in queens and 25.6 ± 4.2 µm in workers (no reliable measurements 118 could be made in males), and have spherical nuclei. Their occurrence in the three 119 castes and location near the junction between the trochanter and femur as well as 120 the opening site of the ducts through the articulation membrane possibly indicate a 121 lubricant function. 122 As no gland cells with this location have been found so far in social insects, we 123 consider this trochanter-femur gland as a novel exocrine structure. The only class-3 124 cells that have been reported in this region occur in the distal part of the trochanter of 125 meliponine bees, although their opening through the sclerotized trochanter cuticle 5 126 (Cruz-Landim, 2002) is different from the intersegmental opening of the novel gland 127 described here. 128 3.3. Leg tendon gland 129 Along their entire length, the legs display a hollow tendon that functions as a 130 claw retractor.