ON THE BRITISH SPECIES OF SIMULIUM — II. 225 



numerous S. aureum on tree-roots in streamlet. Siijfolk : River Lark, near Milden- 

 hall, 25.iv.16, in large numbers on various water-plants, chiefly Ranunculus. Kent : 

 Bushyruff, Dover, 21.iii.l898 (B. L. Hesse). Wales : Anglesey, 19.ix.l5, on stones 

 in small stream across field {C. B. Willia?ns). Channels Is. : Guernsey, Christmas 

 1910, all stages together {K. G. Blair). Scotland : near Edinburgh, 27.ix.15, on grass 

 at edges of small stream flowing through fields ; current flowing at rate of 6 yards 

 in 10 seconds {W. Evans). 



Variation. — In some localities the larvae of S. ornatum are fairly constant, in 

 others they vary greatly in colour. Thus among 4,000 larvae from Mitcham, roughly 

 examined, no variation was noticeable, while specimens from Pegsdon, Herts., 

 and Harefield, Mx., varied in general colour from almost white to dark greenish, 

 blackish or reddish; occasional specimens have dark brown markings as in S. aureum. 

 The head markings are usually distinct, but may be obsolete ; very dark specimens 

 sometimes have the head almost all black. The larvae found in small streams seem 

 to exhibit much more variation than those found in rivers, the greatest variation being 

 shown by parasitised larvae. This may possibly indicate that S. ornatum is 

 normally a river species, the variation being induced by the change of conditions 

 to those found in small streams. 



The pupae exhibit considerable variation in the length of the stalks of the pairs 

 of respiratory filaments, but only in one specimen, out of hundreds examined, was 

 any divergence seen in the number of filaments. This specimen had seven filaments 

 on one side, eight on the other. 



The adults vary in size, and as is the case in some other species, the spring broods 

 seem to be larger than the summer ones, though this is not always very marked. 

 The male varies to a remarkable extent in the silvery markings on the thorax, but 

 only individually ; a close study of the variation might however possibly indicate 

 the existence of definite local or seasonal forms. One such form has been recognised 

 in the New Forest, and is described below. Most males have the thoracic markings 

 similar to those of the female, and no more conspicuous ; some have conspicuous 

 rounded silvery spots much as in the males of S. reptans, etc., while I have seen one 

 or two beautifully marked examples with the whole of the margin of the mesonotum 

 silvery, as well as the usual spots. All these are structurally typical; the only 

 other character which I have noted to vary to any extent is the amount of black on 

 the hind tibiae. The chief variation in the female is in the amount of white on the 

 hind margins of the abdominal segments, which is sometimes very conspicuous, 

 outlining three blackish areas on each segment. To this form Curtis' name 

 trifasciatum will apply. It is evident that S. ornatum is at present a plastic species 

 some derivatives of which have attained stability, while others are probably now in 

 process of development and fixation. It is therefore not surprising that the American 

 representative {S. fiscicidium) should appear to be specifically distinct. 



Blood-sucking. — Since my previous notes on this subject were published, I have 

 had abundant evidence that this species is a blood-sucker, but it would appear not 

 to bite except on warm and rather still days, chiefly in the afternoon sun. Mr. C. L. 

 Walton sent me some specimens from Crosswood, Cardiganshire, which he took in 

 August 1916 attacking horses about the nose and eyes. I have also taken S. ornatum 



