65 



which for a fresh supply of food, should tempt it to cross this 

 dangerous path. First one ant more vigilant than the rest would 

 rush to the attack ; then another, and another, until the poor cater- 

 pillar, entirely covered by its pigmy foes, and completely exhaust- 

 ed in strength by its unavailable efforts to escape, was finally 

 obliged to succumb to superior numbers and die as quietly as pos- 

 sible, when the carcass was immediately carried off by the captors 

 to their nests, or, when too heavy to be dragged away at once, they 

 fed upon it as it lay in the road. This warfare was carried on 

 every day as long as the grass worms prevailed, and no doubt their 

 numbers were diminished in this way to a considerable extent." 



By Riley the occurrence of considerable numbers of a Tachina 

 parasite was noted in 1870 ; and we, ourselves, noticed the eggs of 

 such a parasite this fall on at least fifty per cent, of the worms col- 

 lected by us, most commonly fastened near the head. The imago of 

 one of these parasites emerged in a breeding cage on the 28th Novem- 

 ber, and believing it new, I sent it to Dr. Williston, of New Haven, 

 Connecticut, who, at my request, kindly furnished me the following 

 description : 



Exorista infesta, sp. n., Williston. 



"Female. Palpi reddish yellow at the tip ; third joint of the antennae 

 about thrice as long as the second ; a single row of bristles extends 

 from the front a little ways on the sides of the face above ; the bristles 

 of the face below^ do not reach the middle ; legs black ; scutellum 

 yellowish, the tip of the abdomen reddish yellow. Length 6-7 mm. 



Black. Face grayish white ; on each side of the lower part of the 

 face in front there are but three or four minute bristles above the 

 large marginal bristle. Antennae black, the third joint about three 

 times as long as the second, of nearly equal width, straight on its 

 front margin. Front light ochraceous on the sides, in the middle 

 with a rather broad,, parallel, opaque black stripe, reaching from 

 the ocelli to the base of the antennae; on the lower part there is a 

 single row of not very stout bristles on each side, extending but a 

 little ways on the face. Palpi black, the distal end reddish yellow. 

 Thorax light grayish pollinose, the dorsum showing four rather 

 narrow, black stripes, indistinct behind. Scutellum obscure yellow, 

 or reddish yellow, narrowly black at the base. Tegulffi yellowish 

 white. Abdomen black, the second and third segments very indis- 

 stinctly reddish on the sides, the fifth segment almost wholly red- 

 dish yellow ; second segment opaque ; third and fourth shining, 

 broadly whitish pollinose in front, but variable in different reflections. 

 Legs wholly black. Wings hyaline; last section of fourth vein 

 moderately curved inwards, posterior cross vein gently sinuous. 



Male. Front narrow above, the median black stripe narrower than 

 in the female, and gently widened in front. Palpi black at the base. 

 The sides of the third and fourth abdominal segment broadly reddish. 

 Otherwise as in the female. 



Three specimens : one female (No. 5422), bred by Professor Forbes 

 from Laphygma frugiperda, and a male and a female, collected by 

 Mr. Eugene Keen in Fairmont Park, Philadelphia. The females 



