568 SUMMARY OP OUBRENT RESEARCHES RELATING TO 



The antennre of insects are shown by their innervation to correspond 

 to the first pair of crustacean antonnfc ; the bilobed upper lip of insects 

 is innervated from the second division of the supra-cesophageal ganglion 

 which forms part of the circumoesophageal commissui-e. In the Nauplius- 

 Btage, the second pair of crustacean antennse is innervated from tho 

 circumoesophageal commissure, and a comparison may fairly be drawn 

 between the paired upper lip of Insects, and the second pair of crustacean 

 antennae. Mr. Bruce regards the antenna) of the lusecta and Crustacea 

 as probably homologous structures which ally the two groups. 



The amnion of Insects and Arachnids is probably homologous and 

 allies the two groups, but they and the Crustacea may not have arisen 

 one from the other, but each independently from a common source. Tho 

 trachefe of Insects and Arachnids are probably analogous, not homologous, 

 structures ; this may be concluded from the fact that the trachese of the 

 latter are derived from the lung-books, which are involuted appendages. 



a. Insecta. 



Pol3rpody of Insect Embryos.* — Prof. V. Graber considers that the 

 abdominal appendages which are found on the germinal stripe of various 

 Insects, and which in their mode of development, completely resemble 

 the typical or thoracic legs, are homologous with them. These embryonic 

 abdominal appendages have been most accurately observed in certain 

 Orthoptera, such as GryUotalpa, Mantis, and Blatta, Neuroptera as 

 Neophalax, and Coleoptera as Hydraphihis and Melolontha. In most 

 cases they are only found on the first segment of the abdomen, but in 

 some forms they are also found on the second, and even (in rare cases) 

 on the third. Melolontha is the only form in which they have been 

 found on all except the last two or three segments, but it is not im- 

 probable that polypody, or, better, pantopody obtains in Hydrophilm 

 and the Bee. 



The abdominal appendages are always unjointed, and, as compared 

 with the thoracic, quite rudimentary ; those of the first segment appear 

 simultaneously, or almost so, with those of the thorax, but the others, 

 when developed, only appear later. With the possible exception of the 

 appendages seen by Kowalevsky in Lepidoptera, they are all confined to 

 the embryonic period. Even within this the length of their existence 

 varies considerably, and the hinder appendages are very transitory. The 

 extent and mode of development of the first pair also vary a great deal ; 

 they may either undergo a gradual reduction, or be converted into flat 

 saccules filled internally by loosely arranged cells, which, by constric- 

 tion at the base, become attached to the body by a hollow stalk. In 

 most cases the saccules are only one-third of the length of the legs, but 

 in the Cockchafer they cover nearly the whole of the ventral surface. 



The conditions under which these organs appear make it probable that 

 they are merely the remnants of appendages, or, in other words, that 

 Insects (or Spiders) are derived from ancestors which had well-developed 

 extremities of definite function on their abdomen. These organs were 

 probably all similar, but it may have been that, in adaptation to definite 

 conditions of life, the saccules had the function of gills, or, in other 

 words, the ancestors of Insects and Spiders may have been heteropodous, 

 and been allied to the Crustacea that have posterior branchial sacs. 



* Morphol. Jahib., xiii. (1888) pp. 586-615 (2 pis.). 



