66 



when the mating and egg-laying processes are finished, the females of all three spe- 

 cies have another appearance. The scales of the thorax have almost all fallen off, 

 and the blackish brown colour of the mesonotum especially in lutescens and ex- 

 crucians appears; simultaneously the whole uniform tomentum of the abdomen of 

 0. lutescens disappears, the yellow strongly shining chitin, often a little darker at 

 the base of the segments, appears. The abdomen of excrucians and annulipes gets a 

 rather similar aspect; the scales of the legs and proboscides are torn off; the legs 

 are no more white banded but uniform yellow and only reminiscenses of the whitish 

 rings appear; this more especially holds good with regard to 0. lutescens and annu- 

 lipes. It will therefore be understood that at this time of the year it is impossible 

 to distinguish the three species from each other. As the three species were un- 

 questionably distinguishable from each other in the larva stage, and were conspicu- 

 ously different when newly hatched as imagines, I was inclined to regard them as 

 three species, but was unable to determine them. After having sent them to Dr. 

 EDWARDS I was interested to hear, partly that the separation of the three species 

 was correct, and partly that my material contained the two above-named species 

 0. lutescens and O. excrucians. He wrote to me that he had arrived at the same 

 result as I, that the three species in the female sex are almost indistinguishable 

 from each other later on in the year, after the abdomen has been filled with blood 

 and enormously distended, the females of the three species having lost most of their 

 hair-coating. 



Th6 best distinguishable species is 0. lutescens with its uniform ochraceous, 

 tomentum of the abdomen and the dark mesonotum; also annulipes with its red 

 tomentum, consisting of upright standing hairs is easily recognizable for a long 

 time in the summer; but in very many cases 0. annulipes takes the intermediate 

 stage between O. excrucians and cantans, and the three species are then very diffi- 

 cult to determine. As far as I can see, the male genitalia do not help very much, 

 the structure of the side pieces and clasp-filaments is almost the same in the four 

 species, and only the structure of the harpagos (basal appendages) differs a little in 

 the four species; they are long and strap-shaped in 0. annulipes; in the other three 

 species they are provided with a large, membranous expansion near the tip, and 

 this expansion has a somewhat different appearance from species to species. 



The two species 0. cantans and annulipes have always been difficult to sepa- 

 rate from each other; roughly s.peaking the two species differ from each other by 

 0. cantans being more greyish-dark, O. annulipes more yellowish-red; but with re- 

 gard to the colour most probably every possible intermediate stage may be found. 

 0. annulipes is a more robust species than O. cantans, and this species may occur 

 in pond-races of only very small sizes (from five to six mm.); this I have never 

 seen with regard to O. annulipes. It may be added that O. cantans is a forest mos- 

 quito par excellence, 0. annulipes only found on the large fens and moors; more- 

 over O. cantans appears at a somewhat earlier date than annulipes, and is trouble- 

 some at an earlier time of the year than this last-named species. 



