The structure of the internal Genitalia of some male Diptera. 529 



tion particuliere du bout de rabdomen" (6, p. 266). For the rest he 

 did not make an especial mention of it, whilst Volucella zonaria seemed 

 to be to him a separate case in itself, because he had not observed 

 it in the other Syrphidae examined by him. Nor has Henneguy (8) 

 in his hand-book mentioned this pecuHarity. 



With regard to the general structure of the internal genitaUa, 

 we need only to repeat, what has already been told by Dufoue (5 and 6), 

 Sometimes the accessorial glands are missing {Musca, Dexia, Leptis) 

 but in this case other parts of the genital-apparatus take over the func- 

 tion of these glands. These are e. g. the vas deferens (Leptis) or the 

 ductus ejaculatorius (Musca) or both tubes (Dexia). At the same 

 time we observe that in this case these tubes in proportion are generally 

 of greater length than those of the other Diptera, which are provided 

 with accessorial glands. Yet, besides the accessorial gland- 

 tubes, the ductus ejaculatorius may be secreting either whoUy 

 (Sarcophaga) or partly (Lucilia, Tipula) helping to produce the fluids, 

 which are necessary for liquifying the sperm. 



The testes are pear-shaped bodies, varying from round to oval, 

 which, however, may have other forms, as is proved by the researches 

 of DuFOUR (6). In almost all Diptera they are invested by two layers : 

 the tunica externa and the tunica interna s. propria. The 

 tunica externa is very changeable. It will often tear off from the 

 interna. In most cases it is a brown pigmented cell-layer, which 

 is sometimes thicker ( Syrphus, Rhingia, Musca) at other times thinner 

 (PoUenia, Sarcophaga, Eristalis, Lucilia, Dexia). In other Diptera 

 (Leptis) the tunica externa is unpigmented, exceedingly thin and 

 very difficult to distinguish, whilst sometimes the externa seems to 

 be missing (Tipula, Dicmnomyia). Often tracheal trunks are to be 

 found in the tunica externa. At any rate they are never missing 

 on the outside of the testes; nowhere, however, do the tracheal 

 trunks grow into the lumen of the testes. Even Cholodkovsky (3) 

 has not been able to prove this. However, it does seem to occur in 

 other Insecta, Verson (18) and Tichomirow (17, p. 628) e. g. have 

 observed tracheal trunks in the testes of Bomhyx mori, and Demo- 

 KiDOFF (4, p. 577) in those of Tenehrio molitor. The origin of the 

 pigmentation of the externa is mysterious to me. Brüel (2) asserts, 

 that the tunica externa arises from fat-body-cell-derivations, but 

 he does not know the origin of the pigmentation. In his publication 

 about the ontogeny of the genitalia of Lepidoptera, Zick (21, p. 473) 

 said: "das Pigment ist als abgelagertes Stoff Wechselendprodukt an- 



