The structure of the internal Genitalia of some male Diptera. 507 



into the sacculus. I suppose them to be two small accessorial 

 glands. In fig. 5, which is a transversal section about the height 

 of line z in fig. 1, we see both accessorial glands just before their 

 entrance into the sacculus {sc.acc), while near them, the orifice of 

 the ductus ejaculatorius {d.ef) and the end of the apothetes 

 ejaculatorius (e.ap) are represented. 



Eristalis tenax Linn. (Fig. 1 — 7). 

 The male genitalia of this species almost completely agree with 

 those of Eristalis arhustorum and therefore in the description of Eristalis 

 arbustorum I could refer to the figures from Eristalis tenax. I noticed 

 only a few small points of difference. So I ^vill remark, that the two 

 vasa deferentia are exceedingly thin tubes, which are situated very 

 intimately against the wall of the vesicula seminalis (fig. 6 vd). 

 At the same time I saw in one of my sections in the blind proximal 

 apex of the testes a protoplasmic mass with ramifications (fig. 3) 

 and in its vicinity, nuclei very rieh in chromatin. In these nuclei 

 clear mitotical divisionfigures are to be seen (fig. 3 hr.c). I consider 

 the protoplasmic mass to be a remnant of the apical-cell and the 

 large nuclei to be the mother branch cells of Munson (13). 



Syrphus Ribesi Linn. (Fig. 8 — 12). 



The male gonads of Syrphus Ribesi are formed by one pair of 

 testes with their vasa deferentia, which together form the un- 

 paired vas deferens; the unpaired vas deferens ends, together 

 with two accessorial glands, in the ductus ejaculatorius, 

 which leads out into the sacculus ejaculatorius (fig. 8 and 9). 



As with the preceding species, we find here also the characteristical 

 bending of the caudal part of the abdomen, in consequence of which 

 the terminal part of the abdomen is situated on the right side. This 

 peculiar bending of course causes asymmetry of the internal gonads. 

 In Syrphus Ribesi we also observe a very strong asymmetry in the 

 Situation of the testes with respect to each other, The right testis 

 is always placed moie cranially than the left, whilst the right testis 

 is situated ventro-laterally and the left one, dorso-laterally of the gut. 



Inno way do we see a connection of the testes with the fat-body- 

 tissue. Tunica externa and interna are the only two investments 

 of the testes. The externa (fig. 10 ex) is a thick brown pigmented 

 layer in which large nuclei are to be seen in several places ; these nuclei 

 are always surrounded by a clearer protoplasm. They are so large. 



