38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 80 



muscles of the spiracular closing apparatus were attached to the 

 tergum and the sternum of the same segment. 



Embryologists give us little information concerning the exact 

 position of the spiracles in the embryo with relation to the segmental 

 plates. Lehmann (1925) states that the tracheal invaginations of a 

 phasmid, Caraushts morosus, appear laterally on the bases of the 

 appendages close to the anterior margins of the segments. Heymons 

 (1895), describing the development of Foriicula and Gryllus, says 

 the spiracles arise as pits on the anterior lateral parts of the spiracle- 

 bearing segments, soon after the appearance of the segmental ap- 

 pendages. Wheeler (1889) says the tracheal invagination on the 

 thorax of Leptinotarsa decenilineata are situated at the bases of the 

 legs near the anterior edges of the somites to which they belong; 

 those of the abdomen, however, are placed near the middle of the 

 lateral half of each segment. 



In modern adult insects the location of the spiracles is too variable 

 to serve as an index of what the primitive position of the spiracles 

 may have been, for the adult spiracles may lie in the terga, in the 

 pleural membranes, or in the sterna, and these variations occur within 

 the orders, and often on different segments of the same species. It 

 can be stated as a general rule that the abdominal spiracles lie in a 

 line along each side of the body ; especially is this true of the spiracles 

 of embryos and larvae of most holometabolous insects (Lepidoptera, 

 Hymenoptera, Diptera). In many cases, therefore, where the ab- 

 dominal spiracles of the adult are located in lateral or ventro-lateral 

 areas of the terga, or in lateral parts of the sterna, it would appear 

 that the segmental chitinizations have simply extended from one di- 

 rection or the other over the spiracular areas to include the spiracles 

 in the definitive segmental plates. In other cases, again, it is possible 

 that there may have been a dorsal or ventral migration of the spira- 

 cles, for often the first abdominal spiracle is considerably out of line 

 with those following. In adult Coleoptera, the abdominal spiracles 

 are commonly situated on the dorsal plane of the body, where they 

 may be contained in lateral parts of the terga, in the pleural mem- 

 branes, or in upturned lateral parts of the ventral plates. In the 

 Scarabseidas often the spiracles of the first three or four segments 

 are in the pleural membranes, while those following are in the lateral 

 sternal plates. In adephagous larvae the spiracles (fig. 25, Sp) lie in 

 lateral parts of the dorsum, the ventral limits of which are marked 

 on each side by a lateral fold {a, a) extending through abdomen and 

 thorax. In adults of this group the abdominal spiracles are inclosed 

 in marginal plates of the terga. In lampyrid larvae the si)iracles are on 



