Introduction

During copulation, the ancestors of the Ensifera (Orthoptera) probably assumed the female-above position (Alexander & Otte 1967). This position is still maintained in the recent Phalangopsidae. In this position, the female's head is motionless behind the male's wings, which are held almost vertical, and the female feeds at the secretions on the surface of the male's metanotum (Alexander & Brown 1963, Mello & Reis 1994). The metanotal prominences, where the secretions are found, are called metanotal glands. In this paper we describe findings made by scanning electron microscope (SEM) observations of the metanotal prominences in preserved specimens of Vanzoliniella spp. This technique, using secondary and backscattered electron detection, suggests that there are secretory glands, possibly producing nonvolatile products, below the metanotal sclerites.

Materials and Methods

Hypotheses

In this study we tested if the metanotal sclerites cover secretory glands, forming the observed prominences, and if there are nonvolatile substances secreted by these glands. If there are secretory glands below the prominences, we would expect that on the metanotal surface there would be pores, evidenced by SE. If these glands secrete nonvolatile substances, we would expect that there would be electron-dense regions, exactly below the surface pores, and with a greater diameter than the pores, evidenced by BSE. Nonvolatile substances present elements of high atomic number (Vilela & Della Lucia 1987), resulting in brighter regions in the BSE image in comparison to the cuticular surface. Another evidence for the secretion of nonvolatile substances would be the presence of some kind of residue over the pores.

Results

Vanzoliniella spp. present well-developed metanotal prominences (Fig. 1A,B), with pores throughout their surface (Fig. 1A). There were also pores in the mesonotum (Fig. 2F,H), but occurring more sparsely than in the metanotum (Fig. 2E,G). There were electron-dense regions on the layer below several of the metanotal pores (Fig. 2B), with a greater diameter than the pores themselves (Fig. 2A). Pores with little secretion on the surface presented small electron-dense points, not greater than the pore diameter (Fig. 2A,B). There were also pores without any electron-dense region detected by BSE.

We observed several bright corpuscular bodies (Fig. 2A, C, G, H), apparently being secreted by the pores. These corpuscular bodies included both secretion and ‘dirt’; the secretion could be distinguished by comparing SE and BSE images. We considered as ‘dirt’ any amorphous body in the SE image which did not correspond to an electron-dense region in the BSE image (Fig. 2A–D).

Discussion

The presence of pores on the metanotal prominences confirms that the metanotal sclerites cover secretory glands, and these glands secrete a nonvolatile substance. Even though the analyzed specimens were preserved in alcohol for over 5 y, the secretion was not washed out. This suggests that the secretion is sticky, possibly an adaptive character to prevent it from running off the male's dorsum.

The metanotal prominences appear to be a consequence of the accumulation of glandular tissue below the cuticle. Our work shows that below the surface of most pores there is an accumulation of secretion, with elements of higher atomic number than in the exoskeletal material. This secretion could be stored in two possible ways: superficial glandular structures, or along ducts of deeper glands. As the cuticle of the metanotum forms prominences, we judge the latter hypothesis the most plausible. Examination of cross sections of these regions should elucidate this point. The pores with small or no electron-dense points in the BSE image have probably secreted their content, as evidenced by the absence of secretion in the SE image.

As Vanzoliniella spp. possess neither a stridulatory vein nor auditory tympana (personal observation), the mechanism by which males attract females may involve, for instance, the production of sexual pheromone. Our results do not discount the possibility that such volatile substances are secreted in these or other glands, to attract the females. Olfactometer experiments could evaluate whether these or other male structures are involved in precopulatory courtship.

Entomologists frequently face difficulties in carrying out histological studies, due to the absence of well-preserved or recently collected specimens. The comparative method we present here overcomes these limitations. It can be widely used to evaluate the presence of subsurface structures, such as glands that secrete nonvolatile substances.