NO. 6 INSECT ABDOMEN — SNODGRASS 7I 



There appears to be no reason to doubt that these abdominal 

 appendages of the Protura are remnants of true post-thoracic Hmbs. 

 They have, as Berlese points out, a certain resemblance to the ab- 

 dominal legs of lepidopterous larvae ; but a closer comparison shows 

 dififerences in the segmentation and musculature which makes it seem 

 probable that there is no close genetic relation between the two sets of 

 organs. Prell (1913), in his study of Eoscntomon gennanicum, finds 

 at the base of each abdominal leg two small sclerotizations which 

 he regards as remnants of the subcoxa (fig. 29 A, Sex). The large 

 basal segment he believes is the coxa (Cr) and the smaller distal seg- 

 ment the rudimentary telopodite (Tlpd). The homology of the 

 terminal vesicle (v) is doubtful. The organ does not appear to 

 represent the eversible sacs of Thysanura, since the latter are borne 

 by the limb bases (fig. 4, Vs) ; it might be, however, as Prell sug- 

 gests, the praetarsus, since it has a certain resemblance to the vesicular 

 praetarsus of Thysanoptera. The most likely homologue of the 

 proturan leg vesicles is to be found in the eversible sac on the 

 collophore of Collembola (fig. 30 B, v), which probably represents the 

 united vesicles of a pair of fused appendages. 



GENERAL STRUCTURE OF THE ABDOMINAL APPENDAGES OF INSECTS 



Most of the appendicular organs found on the abdominal region of 

 insects fall into two quite distinct categories distinguished by the 

 insertion points of their muscles. In those of one group the muscles 

 are inserted on the base of the organ ; in those of the other the 

 muscles traverse the organ and are inserted zuitliin its distal extremity. 

 Appendicular structures of the first class are typically stylus-like in 

 form, though they take on various other shapes. They include such 

 organs as the abdominal styli of the Thysanura and the more general- 

 ized Pterygota, the furcula of Collembola, the gills of ephemerid 

 larvae, the terminal claws of trichopterous larvae, the lateral abdominal 

 appendages of larvae of Sialidae, the gonapophyses, the movable 

 claspers of male pterygote insects, and the cerci. Organs of the sec- 

 ond class are sac-like or tubular in form, and are usually retractile and 

 eversible. They include the collophore of Collembola, the eversible 

 vesicles of Thysanura, the gill-bearing tubercles of some sialid larvae, 

 and the plantar lobes of the abdominal legs of larvae of Lepidoptera 

 and chalastogastrous Hymenoptera. 



If we could accept the two categories of abdominal appendicular 

 structures, distinguished by the muscle insertions, as morphological 

 groups of organs, the study of the abdominal appendages of insects 



