NO. 6 INSECT ABDOMEN — SNODGRASS II9 



functions, to which they are adapted by structural modifications. The 

 styliform type is not necessarily the primitive form of the organs. 

 The abdominal styli may be serially homologous with the thoracic 

 coxal styli of MacJiilis, or the latter may be merely large spurs re- 

 sembling the abdominal styli, from which they differ in lacking mus- 

 cles. The abdominal styli are individually movable on the limb bases 

 by muscles arising in the latter. If they are not the rudimentary main 

 shafts of the abdominal limbs, they are exite lobes of the coxae analo- 

 gous with the epipodites of crustacean appendages. They are not 

 comparable with the exopodites of Crustacea, because the exopodite 

 branch of a typical biramous limb is an exite of the first trochanter, 

 or basipodite. 



There are many reasons for regarding the abdominal styli or their 

 derivatives in insects as the rudimentary telopodites of the abdominal 

 appendages. The styli seldom lose their muscles, except when they 

 are immovably united with the bases ; in some insects they take an 

 active part in locomotion ; they may be jointed in a manner sug- 

 gesting at least a true segmentation, and in the larvae of Sialis they 

 have intrinsic muscles in their basal segments. The styli of the gono- 

 pods in male pterygote insects, especially in the holometabolous orders, 

 are commonly modified to serve as grasping or clasping organs during 

 copulation. The styli are the most generally persistent of the distal 

 parts of the abdominal appendages. If it were not for their likeness 

 in apterygote and orthopteroid insects to the coxal spurs of Machilis, 

 it seems doubtful if the abdominal styli would ever have been regarded 

 as anything else than the rudimentary telopodites of the abdominal 

 appendages, represented in a similar form by the cerci on the eleventh 

 segment. 



The vesicles of the abdominal appendages of Apterygota, the gill 

 tubercles of the larva of Corydalus, and the plantar lobes of the larval 

 abdominal legs of Lepidoptera and chalastogastrous Hymenoptera are 

 all organs of a similar and unique type of structure. They are essen- 

 tially exserted or invaginated lobes of the coxal areas of the limb bases 

 lying mesad of the bases of the styli, and are retractile by muscles 

 inserted within their distal parts. In the case of the Apterygota the 

 muscles arise in the limb bases ; in the others they arise from the 

 lateral walls of the body. We might, with Verhoeff, regard these 

 sacs as derivatives of coxal glands, since integumentary glands some- 

 times take the form of eversible and retractile pouches. The coxal 

 vesicles, however, serve a variety of purposes, and they are more 

 simply explained as endite lobes of the coxae, which in some cases 

 have become normally invaginated. They may thus be likened to the 



