34 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The bearings of the characters on the doctrines of natural and 

 sexual selection are discussed in the concluding portions of the paper. 

 It is interesting to note that the Libellulidfe, and especially the 

 larger species, are forms in which the optic organs appear to be re- 

 markably well developed, while their olfactory pits are proportion- 

 ately small in number as compared with other Orthoptera ; so again 

 the plant-eating Hymen optera require less assistance than the saw- 

 flies, and we find consequently, Ichneumon with 5000 pits on each 

 antenna, and Lyda with only 600. In the sexual relations we find 

 similar evidence ; the nocturnal Lepidoptera have the antennae so de- 

 veloped as to form a secondary sexual character, and very much the 

 same obtains in those cases in which the females lead a retired life. 

 One example will suffice — -the male of the Hymenopterous Lophyrus 

 is distinguished from its female by the development of its antennae, 

 and the female is heavy and inactive. 



Structure of the Stigmata of Insects.* — Dr. 0. Krancher, in a 

 preliminary communication, points out that these organs may be 

 grouped under five types of structure : — 

 I. — Stigmata without lips. 



(a.) The simplest stigma is represented by a cleft, which is 

 kept constantly open by a chitinous ring (abdominal 

 stigmata of Diptera, &c.). 

 (6.) The stigma consists of a series of separate stigmata, 

 which are generally surrounded by a common 

 chitinous ring, and which have their tubular pro- 

 cesses united into a single trachea (Dipterous larvae 

 and pupa9). 

 II. — Stigmata with lips. 



(c.) The lips are represented by chitinous pads, simple in 



structure, and not richly provided with hairs. 

 ((Z.) The lips are turned inwards, and are invested by 

 special hairs, which frequently unite to form a close 

 network, 

 (e.) The round stigma has on one side a median piece pro- 

 jecting towards the centre. 

 As to the number of the stigmata, great variations obtain, but 

 never more than ten pairs are found; they are most conveniently 

 divided into thoracic and abdominal stigmata. Of the former there 

 are generally two pairs ; in the Pulicidae, however, there are three 

 pairs. The abdominal stigmata are frequently found on the dorsal 

 surface, and they are always set symmetrically. The position of the 

 orifices seems to depend largely on the habits of the insect. 



The apparatus for closing the tracheae consists of three chitinous 

 bars, moved by a special muscle. Like the stigmata, it presents great 

 variations ; but this much is certain, that it is wanting in no insect. 

 The author regards this apparatus as being morphologically nothing 

 more than a local thickening of the spiral fibre of the trachea ; and 

 this view is supported by what obtains in the simplest cases, where 



* Zool. Anzeig., iii. (1880) pp. 584-8. 



