THE STATE ENTOMOLOGIST. 149 



so easy to explain, but in the present case it is undoubtedly owing to an 

 odor which the larva possesses. This odor is hardly appreciable, when 

 the larvae are in the open air; but by confining a few of them for a 

 short time in a tight box, it soon becomes apparent, and is pungent 

 and nauseous in the extreme even to our sense of smell, and it is 

 doubtless more intensely so to the keener sense of birds and other 

 animals. 



Mr. A. K. Wallace believes that the gay colors of such larvae are 

 really protective, because if by more sombre colors they were undis- 

 tinguishable from edible species, they would be seized by birds, and 

 though rejected afterwards, would be so much injured that the prob- 

 ability of their producing butterflies would be very remote, even if 

 they were not killed outright. 



The same immunity is enjoyed by our Archippus butterfly in all its 

 stages, and especially in the perfect state, in which the peculiar odor 

 is still stronger, as I have abundantly proved. 



The larva does not however enjoy entire immunity from parasites 

 as has been hitherto supposed, for though after extensive experience 

 I have never found any of the numerous Hymenopterous parasites 

 attacking it, it is nevertheless often killed by a Dipterous Ta china- 

 fly. I have never noticed any such parasite in the first brood of 

 larvae, but last year in the immediate vicinity of St. Louis, not one in 

 fifty of the second brood escaped its fatal work; and this same para- 

 site was by no means confined to one locality, as I received it from 

 Mr. S. S. Kathvon, of Lancaster, Pa., who found the Archippus larvae 

 and chrysalids badly infested. The eggs of the Tachina-fiy must be 

 deposited for the most part while the larvae are young, for specimens 

 of larvae taken at the first moult and confined in cages where no flies 

 could get access to them, were frequently parasitised. These victim- 

 ized larvae usually succumb a day or two before they are full grown, 

 though occasionally one succeeds in effecting the change to the chry- 

 salis. They grow sickly and, hanging by the hind legs, become flaccid 

 and discolored, while the parasitic maggots pierce the skin and fall to 

 the ground, which they enter to transform. A silky liquid escapes 

 from the breathing pores and from the holes made by these maggots, 

 which, when dry, forms long white semi-elastic threads; and as the 

 discolored larvae hang by hundreds from the milkweeds, with these 

 glistening filaments, one might at first imagine they had been smitten 

 with some epidemic disease. 



The Tachina maggot is not specially distinguishable from the 

 many other larvae of this kind which are known to infest the bodies 

 of other insects, but the spiracles are encircled by a very distinct dark 

 brown ring.* 



* The larva of this Tachina-fiy, after it enters the ground, contracts very rapidly to the pupa 

 state, and if retained on a hard surface, one may watch with interest how, as the chitinous cover- 

 ing thickens and hardens, the dark head is vigorously kept at work underneath it, gnawing or 

 abrading the thickening skin in a constant circle, so as to partially sever that portion which serves 

 as a lid to be easily pushed open by the future fly. I have often wondered how this lid in so many 



