Comparison of the distribution and nervous innervation of the sensilla on the labrum of Gryllus bimaculatus (De Geer) and Acheta domesticus (L.) (Orthoptera : Gryllidae), and an account of their development in A. domesticus

Abstract

The labra of Gryllus bimaculatus (De Geer) and Acheta domesticus (L.) (Orthoptera : Gryllidae) were studied to reveal the distribution and nervous innervation of the sensilla, in order to provide morphological evidence of their function. Employing scanning electron microscopy and light microscopy, 9 types of sensilla are reported in G. bimaculatus and 10 types in A. domesticus. Of these sensilla, types 1 - 4 have setae ranging in size from 5 - 420 _m in G. bimaculatus and from 3 - 470 _m in A. domesticus. Secretory pores, coeloconic pegs, basiconic pegs and campaniform sensilla have been recorded in both species, while sensilla ampullacea are only present in A. domesticus. The different types of sensilla are found in discrete groups on the posterior surface of the labrum, but they are more randomly distributed on the anterior surface. The detailed innervation of the sensilla was revealed by means of nickel chloride infusion, followed by intensification using Timm's sulphide - silver technique. The labra of both species are innervated by 2 major branches of the labral nerve. Each sensillum is usually innervated by a bipolar neurone, although some sensilla have been shown to be innervated by multiterminal neurones. The dimensions and shapes of the cell bodies and dendrites are provided and differences between the 2 species are identified. In general, the cell bodies and dendrites are larger in G. bimaculatus than they are in A. domesticus. The development of the sensilla through the nymphal instars to the adult is reported for A. domesticus. It is shown that for some types of sensilla, (types 8 and 13), the number remains constant throughout the developmental period, in some (type 1), the adult complement of sensilla is attained in the early instars, for others, (types 2, 6, 7, 9 and 10), there is a steady increase throughout development, while in others (types 3 and 4), sudden increases occur at specific moults.