72 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



would be much simplified. But, unfortunately, there are other fea- 

 tures than the muscle insertions to be considered, such as the origins 

 of the muscles and the position of the organs on the body, that make 

 it doubtful if either constitutes a homogeneous group. Furthermore, 

 organs of each type frequently occur together supported on a common 

 basal structure, and the latter must then be reckoned as an essential 

 part of the primitive appendage, of which the free, movable parts are 

 but accessory structures of one kind or another. Any attempt to make 

 theoretical generalizations on the primitive form or on the homologies 

 of the abdominal appendages of insects will be premature until we have 

 more closely examined the structure of the principal types of such 

 organs as occur in both larval and adult stages of the various insect 

 orders. 



The abdominal appendages of insects fall into three groups corre- 

 sponding with the subdivision of the abdomen into pregenital, genital, 

 and postgenital regions. The appendages of the pregenital segments 

 may be designated the pregenital appendages; those of the genital 

 segment are the gonopods; those of the first and second postgenital 

 segments are conveniently termed the pygopods and uropods, respec- 

 tively. The pygopods are the anal legs, or postpedes, of holometab- 

 olous larvae, and probably the socii and so-called " cerci " of holo- 

 metabolous adults. The uropods are the true cerci. 



THE ABDOMINAL APPENDAGES OF COLLEMBOLA 



The well known appendicular organs on the abdomen of Collembola 

 include organs of each type of structure as defined above according 

 to the muscle insertions. 



The collophore (fig. 30 A, Col) is a large, thick, tubular pouch of 

 the body wall projecting from the sternal region of the first abdominal 

 segment. In most species it ends in a bilobed terminal vesicle (B, v), 

 which is ordinarily retracted but is capable of being protruded by 

 blood pressure. The entire collophore is traversed by a pair of large 

 muscles {rv) arising on the tergal region of the body and inserted on 

 the lobes of the terminal vesicle. The structure of the coUembolan 

 collophore thus suggests that it is formed by the union of a pair 

 of abdominal appendages resembling those of the Protura (fig. 29), 

 though in the latter the retractor muscle (B, rv) arises in the base 

 of the appendage, and the appendage itself is movable by two muscles 

 (/, /) arising in the body and inserted on its base. In some of the 

 Collembola each lobe of the vesicle is produced into a long eversible 

 tube. 



