550 TEXT-BOOK OF ENTOMOLOGY 



The thoracic appendages The three pairs of legs arise at the same 



period and in the same manner in all insects ; it is not until the end 

 of embryonic life that they become jointed, and that the claws and 

 onychia are developed. Especial attention has not yet been given to 

 the details of the development of the parts of the last joint of the 

 tarsus. 



In many forms the antennae are the first to appear, the mandibles, maxillae, 

 and legs appearing at a latter date, though simultaneously. It is thus in Steno- 

 bothrus, Hydrophilus, and Melolontha. In Lina, according to Graber, the man- 

 dibles precede the antennae in appearance. In the Libellulidae, according to 

 Brandt, the legs first appear, then the jaws, and lastly the antennae. This did 

 not seem to be the case in the embryos of -Sschna observed by us, although our 

 observations were more superficial. 



On the other hand, in those insects whose larvae are footless, the rudiments 

 of the legs are retarded and aborted just before hatching (fossorial Hymenoptera 

 and Apidse), or the rudiments of the legs are not developed at all. 



The abdominal appendages. These appear soon after the thoracic 

 limbs, corresponding in most cases to the latter in shape and posi- 

 tion, and their position in the embryo is a matter of the greatest 

 interest. Von Rathke was the first embryologist to detect those of 

 the first abdominal segment, in his examination of the development 

 of Gryllotalpa. Long afterwards Biitschli detected them in the 

 embryo of the honey-bee, observing a pair on each segment. Patten 

 observed them in Trichoptera; Kowalevsky first perceived them in 

 Lepidoptera, Tichomiroff confirming his observations. Graber, 

 Ayers, and Wheeler have observed them in Orthoptera and Coleop- 

 tera, and the latter has detected them also in Hemiptera and 

 Neuroptera; and while they do not arise in the embryos of Diptera 

 and of Siphonaptera, they are to be looked for in any or all the 

 lower or more generalized orders. 



As the result of these discoveries of polypodous embryos occurring 

 in all but the most specialized order (Diptera), it appears to be a 

 rational deduction that the winged insects have descended from 

 insects in which there were functional legs on each abdominal seg- 

 ment. Such an ancestor was the forerunner of the Thysanura, in 

 which abdominal locomotive appendages still survive, though in a 

 modified, more or less aborted condition. This polypodous ancestral 

 form was apparently allied to Scolopendrella, which has a pair of 

 functional legs on each abdominal segment. 



The subject, then, of polypodous embryo insects is one of special interest, 

 and has attracted much attention from Graber, Wheeler, Haase, and others. 

 That these are genuine, though transitory appendages, is shown by the fact that 

 certain pairs persist throughout adult life. The embryology of the Thysanura 



