530 P- E. Keuchenius, 



zusehen." Cholodkovsky's researches on Lepidoptera have caused 

 hini to surmise that the cellular, often pigmented layer of tracheal 

 trunks, sometimes forms flat extending membranes on the testes, 

 and that in this way the tunica externa is formed. He adds to 

 this, that he has come to the conclusion, that some Investments of 

 the testes of Insects are of a "chitinöser Natur" (3, p. 404). Evidently 

 Cholodkowsky's assertion is contrary to that of Brüel (2). I do 

 not know in how far the assertion of Cholodkovsky (3) is to be applied 

 to the tunica externa of the testes of the other Orders of Insects, 

 but it is unfounded in Diptera. In his research about the testes 

 of Diptera he has nowhere observed chitin, whilst I too have been 

 able to indicate in the species, examined by me, the absolute absence 

 of chitin in any of the Investments of the testes. Hewitt too, in 

 his research on Musca domestica wrongly spoke about the chitinous 

 testes (9, p. 432). 



The second Investment of the testes is formed by an epithelium, 

 which as Cholodkovsky (3) has observed, is very rarely developed 

 as such over the whole lining of the testes {Leptis, Tipula, Dicra- 

 nomyia). Towards the blind proximal apex, it generally becomes 

 thinner, loses its cellular nature and continues in the membrana 

 propria. In many Diptera cellular or tissue-septa go out from 

 the tunica propria and push into the lumen of the testes. These 

 septa may be ramified and in general they are not perfect, because 

 they end in the lumen of the testes. In RJiingia rostrata on both 

 sides of such a septum an excrescence of cells has been found. Per- 

 haps it has something to do with the nutrition of the germ -cells. 

 In several Diptera (Tipula, Leptis, Dicranomyia) the testes are 

 enclosed by fat-body-tissue, which has intimately grown together 

 with the whole surface of the testes, whilst in a few [Lucilia, Pollenia), 

 the fat-body is united with the tunica externa only at some places. 

 This intimate connection of the tunica externa with the fat-body 

 is, according to Brüel (2, p. 514), the consequence of the fact, that 

 the tunica externa has arisen from the fat-body-cells. However, 

 it is pecubar, that in the most examined Diptera, there is no question 

 of any connection of the testes with the fat-body, but it may be 

 possible, that this connection has existed in the larva-stage, after- 

 wards ceasing in the adult. 



In my opinion, the fat-body -tissue cannot in any way be 

 considered as an investing layer of the testes. 



Peculiar is the case in Lapliria, described by Dufour (6, p. 244) 



