NOTES ON THE STRUCTURE OF SILK GLANDS IN SPHECID WASPS, MICROSTIGMUS AND PSENULUS (HYMENOPTERA, SPHECIDAE, PEMPHREDONINAE)

by

JOSÉ EDUARDO SERRÃO* and LÚCIO ANTONIO DE OLIVEIRA CAMPOS (Departamento de Biologia Geral, Universidade Federal de Viçosa, 36571-000 Viçosa, MG, Brasil. E-mail: [email protected])

ABSTRACT

The silk glands of adult sphecid wasps, Microstigmus and Psenulus were examined by light and scanning electron microscopy to describe their structure. Silk glands in both genera were classified as epidermal glands of the type 3, that is they are formed by one multinuclear terminal cell. From the terminal cell, one conducting canal runs along the body cuticle. The canals open to the exterior by means of spinnerets. These cuticular structures have different positions and forms in Microstigmus and Psenulus. The similarities in the structure of silk glands and the differences in spinnerets between these two genera confirm previous reports that silk glands evolved independently in Sphecid wasps. KEY WORDSHymenoptera, : silk glands, Sphecidae, sphecid wasps.

INTRODUCTION

Production of silk-like materials is widely distributed among Arthro- poda, especially Chelicerata and Insecta. Among insects, holometabolous groups present many species that synthesize silk in larval forms, e.g., some Lepidoptera, Hymenoptera, Coleoptera, Neuroptera and Diptera (MICH- ENER, 1974; KENTCHINGTON, 1983; SEHAL & AKAI, 1990). How- ever, production of silk in adult insects is restrict to a few species of Hymenoptera, Psocoptera, Hemiptera, Diptera and Embioptera (WEBER, 1930, RICHARDS & DAVIES, 1977, SEHAL & AKAI, 1990, NAGASHIMA et al., 1991; MELO, 1997). Among Hymenoptera, production of silk in adult forms has been re- ported for Chalcidoidea (DELANONE & ARAMBOURG, 1965; WOOLEY & VET, 1981), Vespidae (SINGER et al., 1992) and Sphecidae (MELO,

* Author for correspondence. 480

1997). In sphecid wasps, the silk produced by female members of the sub- family Pemphredoninae is used in nest construction (MELO, 1997). Ac- cording to this author the silk glands are situated in the metasoma, opening to the exterior by means of bristles acting like spinnerets in the formation of silk threads. As part of an ongoing study, the present paper deals with the morphol- ogy of the silk glands in sphecid wasps Psenulus and Microstigmus ob- served under light and scanning electron microscope.

MATERIALS AND METHODS

The material used in this study consisted of adult females of Microstig- mus bra.siliensis, Microstigmus nigrophthalmus and Microstigmu.s arlei captured in Viqosa, MG, Brazil. Psenulus sp. 1. (aff. aztecus) and Psenu- lus sp.2. (aff. aztecus) from Costa Rica were supplied by Dr Gabriel A.R. Melo. Voucher specimens are deposited in the Central Apiary of the Universidade Federal de Viqosa. For each Microstigmus species three specimens were analyzed and for Psenulus species only one. For light microscopy the metasoma (abdomen), after having been carefully cut in the lateral middle line, were fixed in 4% paraformaldehyde in phosphate buffer 0.1 M, pH 7.2. The pieces were then thoroughly rinsed with buffer, dehydrated in ethanol series and embedded in historesin JB-4 (Polyscience). Sections 4 pm thick were obtained with a glass knife, stained with haematoxylin and eosin or Domici solution. Some sections were treated with Feulgen reaction. For scanning electron microscopy (SEM) opened metasoma were fixed in the same fixative solution as for light microscopy, and dehydrated in an ethanol series. The pieces were then immersed in hexamethyldisilazan (HMDS), air dried at room temperature and gold coated. Specimens were examined with a Jeol SEM.

RESULTS AND DISCUSSION

In five species analyzed many individual glands could be seen scattered in the more posterior portion of the metasoma. Each gland is formed by an enlarged secretory subunit (terminal cell) with one conducting canal (fig. 1). These canals are grouped in bundles lining the body cuticle and are open to the exterior by a number of cuticular processes, named spinnerets (fig. 2). The number of conducting canals that open in each spinnerets could not be determined. However, the greater number of