The Scottish Naturalist. 119 



half fed, and in this case the struggle for existence must be 

 harder, but the death of the host seems always to happen. 



The larvae of B. brassicce, themselves almost parasites, are 

 preyed upon by at least two Hymenopterous insects. 



On the 26th of July, a larva of a small Ichneumon was 

 observed crawling over one of the lafvas, which was then quite 

 lively and evidently uninjured. The ichneumon soon fastened 

 its mouth in the other's side, and I watched it, by the aid of a 

 lens, evidently sucking it with great gusto, and giving now and 

 again a twist to its body, as if wanting to get into a better 

 position to enjoy its meal. While looking at it, I was disturbed 

 and let the two fall to the ground, a distance^ of about four 

 feet, and on immediately picking them up was interested to 

 find the parasite in the same spot, and sucking away as vigorous 

 as ever. By the 28th it had devoured the tail half of the body ; 

 on the 30th the rest of it was gone with the exception of the 

 mandibles, and the parasite had spun up in a white cocoon. 

 Even when full fed it was considerably smaller than the weevil. 

 The dorsal vessel was of a reddish colour. 



A still smaller insect, one of the Chalcididce, is the other 

 parasite. Although equally voracious it takes a longer time to 

 finish the larva, and like the other sucks it from the outside. 

 It does not spin a cocoon. 



Both these insects feed likewise on the larvae of the saw-flies. 



Acentropus nivsus— How to find and capture it.— I would advise any 

 entomologist who wishes to become acquainted with the history of this very 

 peculiar insect, to peruse Mr. Dunning's elaborate essay on the subject, which 

 is published in the "Transactions of the Entomological Society of London, 

 1872." I am under the impression that the species is much more generally dis- 

 tributed in Britain than is generally supposed, and that on account of its small 

 size and insignificant appearance, it is often overlooked. I purpose, therefore, 

 to give a few hints how to find it. In the first place, it must be remembered 

 by those who lcok for it, that the early stages of its life are passsed in the 

 water, and that, consequently, the perfect insect is seldom found far from that 

 element. It is found from the beginning of June till September with us in 

 the south. The principal food of the larva are the various species of Potamo- 

 geton, but from recent accounts it appears the moths have been taken abundantly 

 where Potamogeton was comparatively rare, but the American water-weed Ana- 

 char is alsinastrvm was common. There was no proof, however, that the larva fed 

 on the latter. I have found the moths most commonly in July. They begin to fly 

 just about dusk close to the surface of the water, and seem to be swimming 

 about in broken circles, and often glide out upon the mud at the margin of the 

 water. Though they are very small, their white appearance makes them very 



