Fig. 271.—Longitudinal section of part of cercus of Acheta domestica: ch, cuticula; hyp, hypodermis; n. nerve; h′1, integumental hairs, not sensory; h2, ordinary hair; h3, sensory hair; h4, bladder-like hair; sz, sense-cell.—After Vom Rath, from Sharp.
In the course of a special description of these sense-organs in the Orthoptera, Hauser describes at length those of Œdipoda cœrulescens and Caloptenus italicus. On one antennal joint of Caloptenus (Fig. 268) was often counted 50 pits; on the anterior joints the number diminishes to about 30. Hauser thinks that in all Orthoptera whose antennæ are like those of Caloptenus occur similar pits, as he found them in Stenobothrus (Fig. 269) as well as in Œdipoda. Gryllotalpa possesses similar pits,—four to six on each antennal joint, making between 300 and 400 pits on each antenna.[[48]] In Mantis religiosa the pits were not detected, but on each joint, except the eighth basal, there are about 200 small, hollow, curved teeth with a fine opening in front.
In the Neuroptera (Chrysopa) there occur on the antennæ, besides numerous very long tactile bristles, small pale, transparent teeth. No pits could be detected.
In the Hemiptera (two species of Pyrrhocoris only were examined) only two kinds of tactile bristles occurred, but Hauser detected no pits, though Lespès states that they are present.
Fig. 272.—Longitudinal section of apex of palpus of Pieris brassicæ: sch, scales; ch, cuticula; hyp, hypodermis; n, nerve; sz, sense-cells; sh, sense-hairs.—After Vom Rath, from Sharp.
Of the Diptera, Hauser examined more than 60 species. The pits in the Diptera brachycera (Muscidæ, etc.) are unexceptionally confined to the third antennal joint. Their number varies extraordinarily in the different species. Helophilus florens has on each antennal disk only a single pit, while Echinomyia grossa possesses 200 of them. In flies of certain families the pits are compound, and contain 10, 20, and often 100 olfactory hairs, partly arising from the coalescence of several pits. Such pits are usually divided by lateral walls into several chambers, whose connection is only indicated by their common outlet. Simple olfactory pits with a single olfactory style were observed only in the Tabanidæ, Asilidæ, Bombylidæ, Leptidæ, Dolichopidæ, Stratiomyidæ, and Tipulidæ. In the last the compound forms do not occur at all, but in the other families mentioned also occur compound pits, receiving from two to ten nerve-terminations.
The antennal pits of flies are always sac-like invaginations of the external chitinous integument, of manifold shapes, opening externally and never closed by a membrane. The pits differ but slightly in the different species, and that of Cyrtoneura stabulans (Fig. 273) is described at length as typical of those of brachycerous flies in general.
The olfactory pits of the Tipulidæ seem to have a somewhat different structure, since the external passage is closed. It is circular, surrounded with a slight chitinous wall, and not covered with bristles. Such pits in their external appearance are like those of the locust (Caloptenus) and many Hymenoptera. They are situated usually on the third antennal joint. Pachyrhina pratensis L. has about 60 of them, as have Tipula oleracea L. and Ctenophora.