APPENDAGES OF FIRST ABDOMINAL SEGMENT 551 



when worked out will throw much light on this subject, but we know that the 

 spring (elater) of Collembola (and possibly the collophore) and the cerci of the 

 winged insects are survivals of these limbs. That the three pairs of appendages 

 forming the ovipositor, or sting, are most probably derived from these appendages 

 is claimed by Wheeler (p. 167), and seems proved by the fact that Ganin and 

 also Bugnion has detected three pairs of imaginal disks in the embryo of para- 

 sitic Hymenoptera. Hence the abdominal appendages may ultimately be found 

 to arise in nearly all cases from imaginal disks like those giving origin to the 

 cephalic and thoracic appendages. 



As regards the Diptera, Pratt has observed that each of the three thoracic and 

 eight abdominal segments of the embryo brachycerous Diptera (seen especially 

 well in Melophagus) has two pairs of imagiual disks, a dorsal and a ventral pair. 

 He thinks there is no doubt but that the ventral abdominal disks are homologous 

 with the rudimentary appendages which appear in the embryos of all other 

 insects, though not in the brachycerous dipters. 



Appendages of the first abdominal segment (pleuropodia). As early 

 as 1844 Rathke observed in the embryo of the mole-cricket a pair 

 of appendages on the 1st abdominal segment, which he described 

 as mushroom-shaped bodies, and supposed to be embryonic gills. 

 They are called pleuropodia by Wheeler, who, with Patten, Graber, 

 and Nusbaum, ascribes a glandular function to them, while Wheeler 

 suggests that they were odoriferous repugnatorial organs. In Blatta 

 (Phyllodromia) they are of large size, in Melolontha enormous (Fig. 

 528, J5) and filled with blood. Wheeler distinguishes as varieties, 

 beside the mushroom-shaped appendages of Gryllotalpa and Hydro- 

 philus, the reniform (CEcanthus), the broadly pyriform (Blatta), and 

 the elongate pyriform (Mantis Carolina). In the European IMantis 

 they are most limb-like, with a digitiform continuation divided by 

 a constriction into two sections. (Graber.) In Meloe they assume 

 the shape of a stalked cup. (Nusbaum.) In the bee and in Lepi- 

 doptera the pleuropodia are not present, though the temporary 

 appendages on the succeeding segments appear; Carriere, however, 

 found them on the two first abdominal segments of very young larvae 

 of the wall-bee (Chalicodoma). 



Their cellular structure is peculiar, and they are either formed 

 by evagination or invagination, those of the latter type being sub- 

 spherical and solid. Those of the former type have a cavity com- 

 municating by means of a narrow duct through the peduncle with 

 the body-cavity (Blatta). No tracheae, nerves, or muscles enter 

 them, though blood-corpuscles have been seen in the cavities. " In 

 some species the pleuropodia produce a secretion from the ends of 

 their enlarged cells. This secretion may be a glairy albuminoid 

 substance (Cicada, Meloe), a granular mass (Stenobothrus), a bun- 

 dle of threads (Zaitha), or a thick, striated, cuticula-like mass 



