BOTH THE SEXES IN DIPTERA. 363 



transverse, setose plates, and three long narrow rods longitudinally support the 

 membrane (PI. 29. fig. 107). 



In CalUphora erythrocephala there is a dorsal plate opposed by a shorter ventral plate. 



PoUetes Icu^daria has three transverse setose plates. 



Z^lidia nlg^'ipennis has four half-formed chitinous rods on this segment (PI. 28. fig. 95). 



Before discussing the nest section, " the spiracle-bearing segment," I propose making 

 a few remarks upon a part that is occasionally met with and whose presence accounts 

 for traces of chitinous structure found without any apparent reason in the ovipositor. 



The Aiwdeme in the Ovipositor. 



Simuliidce. — In Siinidium reptans (Linn.) * tliere is an apodeme at the extremity of the 

 abdomen ; it is a thin chitinous process and forks at its posterior part, the forks partially 

 enclosing an opening (which appears to be that of the vagina) and having at their 

 extremities some connexion with the external valves. The function of this part is to 

 rotate the opening, as I have preparations which show the shaft of the apodeme in both 

 anterior and posterior relation to the opening. 



AsiUdcB. — In an undetermined Asilid from Queensland there is, in a similar position, 

 a plate with tiie posterior extremity furcate ; near this part the orifices of the ducts of 

 the three receptacula seminis open. This apparatus appears to be homologous with 

 the apodeme in Simulium, and explains the frequent appearance of chitinous rods in 

 Sijritta (Syrphidae), Stratiomt/s (Stratiomyiidse), and other insects. 



Somologij. — In homologizing this part with the ovipositor of Musca domestica, its 

 furcation suggests that it is represented by the partially fused rods in the third segment, 

 but its position points to other structures in a more anterior position. It appears to me 

 that these levers are a development in the direction of the telescopic ovipositor, it being 

 of advantage to insects to extrude the ovipositor even in a small degree. 



Chironomidce. — Chitinous structures are also found close to the valves of Chironomus 

 plumosus (Linn.) and C. rlparius, Meig. 



The Spiracle-bearing Segment. 



Bibionid(C. — In Bihio hortulanus, Bilophus febrilis (Linn.), B. albipennis, Meig., and 

 Scatopse notata the abdomen has, on the segment next the penultimate segment, single 

 paired spiracles, as on this part in those families where the ovipositor is telescopic. 



Stratiomyiidce. — In Beris vullata the spiracles arc on the membranes on the sides of a 

 central, ventral, smaller plate. In B. nigra the plate is large, consequently the spiracles 

 are farther apart. 



Tabanidce. — In this family, unlike the Bibionidse, the spiracles appear to be on the 

 penultimate segment; but I think, from the evidence of a small triangular piece in 

 Hcematopota pluvialis (Linn.) and H. cra^sicornis, Wahlb., that the last segment has 

 fused with the next and represents two. 



Bmpidce. — In the Empidte the spiracles are about the middle of the segment. 



* There is some uncerlaiuty as to this insect, but it is of the same size and colour, and if it is not the genuine 

 /S'. reptans, Linn., it is almost impossible without special study of this family to separate the two species. 

 SECOND SEllIKS. — ZOOLOGY, VOL. IX. 53 



