REPORT OF THE ENTOMOLOGIST. 511 



DESCRIPTIVE. 



There are some characters which these two species possess in com- 

 mon with all other species of the genus, though scarcely any of the 

 described species are known in both sexes. It may be well to state, 

 therefore, that the male differs markedly from the female in his much 

 smaller abdomen and relatively larger thorax, by the mouth parts 

 being soft and subobsolete, and more particularly by the e^^es being 

 confluent and having two well-marked and distinct sets of facets. 

 As we have already stated, the male is not found flying with the 

 female, and we should not have obtained this sex in the two species 

 here treated of had they not been bred from the larvae. It is desira- 

 ble to describe both sexes from fresh and living specimens, as they 

 become sordid in alcohol, and shrink and lose much of their charac- 

 ter and color when mounted dry. The females are also somewhat 

 altered in appearance after haA^ng been gorged with blood. The pro- 

 thoracic is the only spiracle traceable in the insects of this genus. 



The larvse of all the species known have very much the same gen- 

 eral form and structure, and they differ chiefly in some of the details 

 of the flabelliform fan and of the mouth parts. 



The pupa in form foreshadows that of the future fly, and the spe- 

 cies differ in this state chiefly in the number of filaments or ramifi- 

 cations thereof that compose the breathing organs. These are inva- 

 riably situated, one on each side, upon the anterior dorsal mar- 

 gin of the thorax, each originating in a single trunk, which soon 

 branches into rays which are fine hollow tubes, apparently composed 

 of rings, and closed at their extremities. Each tube consists, further, 

 of one or two chitinous layers covered by a finely granulated mate- 

 rial. In both the species under consideration there are two of these 

 chitinous layers, of which the inner is very thin and smooth, the outer 

 thicker and furnished with pores. The base of the trunk connects 

 by a stigma-like ring with a true spiral tracheal tube visible beneath 

 the epidermis, and which, bending suddenly inwards, contracts and 

 connects with the internal tracheal system of the corresponding side.* 

 At the tip of the last abdominal segment, upon the dorsal surface, 

 are two hooks, which engage in the meshes of the cocoon, to hold the 

 pupa in position. Some few threads of loose silk and the old larval 

 skin are also found in this situation. Minute black hooks, arranged 

 in regular and definite order upon the dorsal and ventral surface of 

 the abdomen, assist the pupa to keep its position inside the open co- 

 coon. These hooks are usually bent upwards. 



The cocoons of the various species differ from each other both by 

 their structure and by the method by which they are fastened to 

 plants, stones, &c. Generally speaking, the cocoon is a brownish, 

 obconical, semi-transparent pouch, open above, more or less covered 

 with mud, and directed against the current of the water. The pupa 

 is more or less tightly surrounded by it, and has the anterior portion 

 protruding above the rim of the pouch. The cocoons are formed of 

 irregular threads, which harden rapidly in the water, and in the 

 deeper parts of the cocoons there are also some long loose and dis- 

 connected threads. 



* Dr. Voccel. in his description of the tracheal tubes of the pupae of SimuUum, 

 gives a similar description, stating that, contrary to the published opinion of Siebold, 

 there are no tracheae inside the tubes. — Mittheiliingen der Schweizerischen Ent. Ges. . 

 Vol. VII, Heft 7. 



