412 INSECTA. 



dorsal border of the antennae. This pit is at first provided with 

 a lumen, which is subsequently obliterated ; while the walls of 

 the pit become converted into true ganglion cells. The two 

 supra-cesophageal ganglia remain disconnected on the dorsal 

 side till quite the close of embryonic life. 



The tracheae and salivary glands. The tracheae, as was 

 first shewn by Butschli (No. 405), arise as independent segment- 

 ally arranged paired invaginations of the epiblast (fig. 189 B and 

 C, st). Their openings are always placed on the outer sides of 

 the appendages of their segments, where such are present. 



Although in the adult stigmata are never found in the space 

 between the prothorax and head 1 , in the embryo and the larva 

 tracheal invaginations may be developed in all the thoracic (and 

 possibly in the three jaw-bearing segments) and in all the 

 abdominal segments except the two posterior. 



In the embryo of the Lepidoptera, according to Hatschek (No. 414), 

 there are 14 pairs of stigmata, belonging to the 14 segments of the body 

 behind the mouth ; but Tichomiroff states that Hatschek is in error in 

 making this statement for the foremost post-oral segments. The last two 

 segments are without stigmata. In the larvae of Lepidoptera as well as those 

 of many Hymenoptera, Coleoptera and Diptera, stigmata are present on all 

 the postcephalic segments except the 2nd and 3rd thoracic and the two last 

 abdominal. In Apis there are eleven pairs of tracheal invaginations accord- 

 ing to Kowalevsky (No. 416), but according to Butschli (No. 405) only ten, 

 the prothorax being without one. In the Bee they appear simultaneously, 

 and before the appendages. 



The blind ends of the tracheal invaginations frequently (e.g. 

 Apis) unite together into a common longitudinal canal, which 

 forms a longitudinal tracheal stem. In other cases (e.g. Gryllo- 

 talpa, DoJirn, No. 408) they remain distinct, and each tracheal 

 stem has a system of branches of its own. 



The development of the tracheae strongly supports the view, 

 arrived at by Moseley from his investigations on Peripatus, that 

 they are modifications of cutaneous glands. 



The salivary and spinning glands are epiblastic structures, 

 which in their mode of development are very similar to the 

 tracheae, and perhaps have a similar origin. The salivary glands 



1 In Smynthurus, one of the Collembola, there are, according to Lubbock, only 

 two stigmata, which are placed on the head. 



