214 F. W. EDWARDS. 



anal sucker — in S. latipes about 15 mouth-brush hairs and about 4 teeth in each 

 sucker-row ; and by the apparently two-jointed antennae, the second joint being the 

 minute cone-shaped one. The later stages differ from one another chiefly in the 

 size of the head, and in the prog^ressively greater number of hairs in the mouth-brush 

 and teeth in the sucker-rows. The full-grown larva of >S'. latipes has about 40 haiis 

 in the mouth-brush and 12-15 teeth in the sucker-rows. The anal gills in this species 

 are quite simple in the first three larval stages, very slightly branched in the fourth, 

 and distinctly so in the last. In this species, as in several others, the head is much 

 darker in colour in the younger larvae, and the markings are less distinct. 



Diagnostic Characters of Larvae and Pupae. — It is not the; writer's intention to 

 describe the early stages in any detail, but merely to indicate the main points of 

 difference between them. In the larvae these are to be found chiefly in the number 

 of joints in the antennae, in the form of the anal gills, in the markings of the head, 

 and to some extent in the colour of the body. The structure of the mentum and the 

 number of hooks in the anal sucker also vary much with the different species, and 

 some use has had to be made of these points. The mandibles and maxillae certainly 

 exhibit good specific characters, but these have been ignored, as the more obvious 

 distinctions are sufficient to separate all the British species. In the pupae the only 

 characters of systematic value are the form of the cocoon and of the prothoracic 

 respiratory filaments, but these are often so well-defined that they form a much 

 easier means of identifying the insect than any characters of the adult ; they have 

 the advantage of being entirely uninfluenced by sex, and are subject to very little 

 variation. 



Seasonal Variation. — ^The occurrence of definite seasonal variation in the Diptera 

 is only known in a very few instances. The only two cases, so far as I am aware, 

 in which it is regarded as proved, have been recorded by Hensel* for Dryomyza 

 flaveola, and by Aldrichf for Cerodonta dorsalis. It has also been suggested by Verrall 

 that Tipula oleracea and T. paludosa may be summer and autumn forms of the same 

 species, but this is not definitely established. 



In the case of Simmlium the spring brood of S. equinum is conspicuously larger 

 than the subsequent broods, while there may also be a slight difference in S. ornatum ; 

 but this latter is such a variable species that it is difficult to say for certain. In 

 8. argyreatvm, however, we have a well-marked case of seasonal variation, as both 

 males and females of the spring brood are fairly readily distinguishable by colour 

 and markings, as well as by their larger size, from the later broods ; the spring form 

 is described below as var. sericatum, Mg. For absolute proof that these two forms 

 are really the same species it would of course be necessary to breed one from the other, 

 which I have not been able to do, but still I think the assumption may be justified 

 for the following reasons : — (1) the male genitalia are peculiar and are identical in 

 structure in the two forms, which are also alike in all other structural characters ; 

 (2) in any locality where one form occurs the other may also be found ; (3) the spring 

 foim is never found after about the middle of May, and the summer form never before 

 the beginning of that month. 



* Berl. Ent. Zeit., 1870, p. 1.33. 



t Ann. Ent. Soc, Amer., xi, 1918, p. 63. 



