HYMEXOPTEROUS PARASITES. 267 



extension and retraction depends upon slight telescopic movements of 

 the sclerites permitted by the infolded membranes which connect their 

 adjacent edges. In a greatly exaggerated form this type of construc- 

 tion is by no means rare; it occurs in the apical part of the abdomen of 

 the higher Diptera, in the midge upon which Dolichotrypes is parasitic, 

 in the Serphoid Scorpioteleia, referred to on a later page, and quite 

 frequently in association with the ovipositor of various insects. 



Careful dissections of the abdomen of Dolichotrypes show, however, 

 that only the membranous apical segment is extrusible and retractile in 

 response to muscular impulsion. The basal tubular segments (4, 5 

 and 6) are of a fixed length in each individual insect although one seg- 

 ment may be eight or ten times as long in one specimen as the corre- 

 sponding one in another example. Furthermore, it is impossible to 

 segregate a large series of indi\dduals into classes, based on length of 

 segments as the proportionate lengths are not constant, although there 

 is a well-marked tendency for all to be either long, short, of medium 

 length, etc. 



The reasons for believing that the lengths of the chitinized segments 

 cannot be changed by muscular action are very clear. The individuals 

 do not show any segments in which the chitinized basal end is tele- 

 scoped within the apex of the preceding segment, nor do any of them 

 show an elongation of the intersegmental membranes. In all cases the 

 exposed portions are black and thickly chitinized, but no hardened 

 portions remain concealed. It is evident, therefore, that the seg- 

 mental lengths of adults are fixed and that they have been determined 

 previous to the hardening of the exoskeleton which occurs soon after 

 the insects have undergone their last ecdysis from pupa to imago. 

 AVhether it occurs at the time of pupation cannot be stated definitely as 

 no puppe have been observed, but as the form of such parts is usually 

 determined at that time there is no reason to believe otherwise in this 

 case. It seems probable, therefore, that the ultimate form of the 

 abdomen is determined when the pupa is first formed, after which 

 pigmentation and chitinization develop slowly. 



It is noticeable in specimens with extremely long fourth segment 

 that this segment is just long enough, if it could be retracted within 

 the body, to reach to the anterior region of the thorax as is the case 

 with the ovipositor in Inostemma. ^ Such is also true in most of the 

 long-tailed individuals with regard to the length of the membranous 

 scA'enth segment which when exserted equals approximately the sum 



1 The condition of the ovipositor in this genus is discussed on a later 

 page (p. 280). 



