BOTH THE SEXES IX DIPTERA. 353 



actuated by the apodeme of that part, and the organs under discussion, " fallen from 

 their high estate," are reduced to act as tlie lateral levers of the theca, to which they are 

 fused (PI. 24. fig. 23). In GijnopUstia bella they are, on the contrary, of great 

 importance and development, attached by muscles to the complicated mechanism of the 

 theca, apparently to separately rotate the two central hooked processes (PL 23. fig. 15). 

 In Pachi/rrhina maculosa they are represented by two rather weak chitinous plates, 

 laterally supporting the theca. 



StrathmijiidcB, Asilidce, Umpidce^and Dolichopodidcs. — In Beris vallata, the Asilidae, 

 the Einpidfe, and the Dolichopodidge these levers seem absent, or are fused to the theca, 

 the single ejaculatory apodeme doing all the work. In the undetermined Asiiid, already 

 alluded to and figured, the anterior portion of the theca sends out j)rocesses for the 

 attachment of muscles, and possibly represents the fused apodemes (PI. 24. fig. 38). 



Si/rphidcB.— This part is very diflBcult to make out in the Syrphidse. It appears to be 

 present in SphcBriopJioria scripta, bu.t seems in that case to be doing the work of the 

 ejaculatory apodeme, which I cannot find. There is a suture down the organ and large 

 lateral processes to support the theca. I cannot find it in Catabomba pyrastri, but am 

 not sure of its absence, as in this family, as I shall show, the part migrates in a surprising- 

 manner. In Si/rlita pipiens, in the upper part of the theca, is an arched opening ; this, 

 seen from the front, has some curious and minute structure on its floor, the orifice of 

 the ejaculatory duct. Examiniug the organ from the side, I am able to make out that 

 this floor is the upper part of a chitinous box ; and welded to this box, and working in 

 the centre of the theca, is the great apodeme, quite single and extending downwards to 

 the top of the ejaculatory apodeme, which is very large and evident. From this central 

 position it may rotate the whole organ, but does not seem sufficiently powerful for that 

 purpose (PI. 26. figs. 56, 57). 



In Eristalis tenax there is no domed aperture, but the orifice of the duct is also 

 placed in a chitinous chest or box ; to the front of this box is hinged the apodeme, which 

 rotates it and the two hooks (forcipes interiores) which are attached to it. The whole 

 organ (the theca) is rotated by its edges, which are rounded and thickened, and project 

 at the base, forming levers (Pi. 26. fig. 58). 



Miiscidce. — In the Muscidae the organ is generally at the base of the theca and is 

 fused for its whole length : it takes this characteristic form in Sarcophagu carnaria 

 (PI. 26. fig. 60), in Olivieria lateralis (PL 29. fig. 112), in Pollenia rudis (PL 26. fig. 62), 

 in Calliphora erythrocepliala (PL 26. fig. 68), in Anthomyia radicum (PL 27. fig. 85), in 

 Norellia spinimana (PL 28. fig. 89) and Scatophaga litorea (PL 28. fig. 88), besides the 

 gi'cat majority of the other species. 



In Glossiua there is anotlier remarkable metamorphosis. It has become forked at its 

 upper part as in G. palpaUs (PL 27. fig. 72) and G. tachbioides (PL 27. fig. 75), or has 

 spread out into a plate with an aperture to contain the ejaculatory sac, as in G. morsitans 

 (PL 27. fig. 74) and G. pallidipes (PL 27. fig. 73). It works through the theca, whicli 

 has become a wall surrounding it ; and in G. palpaUs, G. tachinoides, and G. pallidipes 

 the upper ends are articulated on to the paraphalli, which, in their turn, are articulated 

 on to the theca. 



