THE NEW MEXICO RANGE CATERPILLAR. 91 



standing out stiffly horizontal in front of her face. After a minute of silent attention 

 she moved carefully toward the worm, gliding over the intervening grass blades with 

 a stealthy step much unlike the usual walk of these large flies. She maneuvered 

 about the caterpillar with quick, nervous, jerky steps, keeping well below the body 

 of her victim . The abdomen of the fly all the while moved slightly, as if the ovipositor 

 were being made ready. 



At last the caterpillar found it necessary to move to fresh pasture. Its true feet 

 crept along the stem and as the movement swept back through the clumsy body 

 the fly ran nimbly up below, ovipositor extended. As soon as the thick prop-feet 

 were lifted, the ovipositor • curved quickly upward once or twice, but things were 

 not quite right and there was no contact. The fly ran along the grass blades to the 

 larva's head, got in front of it, and once more stood facing it for some minutes. The 

 larva finally seemed to realize its danger and showed its annoyance by a few quick 

 jerks of its head and body, apparently intended to scare away the intruder. But 

 the fly held her ground, with now and then a quick step or motion, followed by 

 rigid quiet. The larva at last became quite uneasy and began to walk off, where- 

 upon the fly again ran below. The ovipositor was thrust swiftly upward several 

 times, but the prop-feet did not seem to open enough to allow of the placing of any 

 eggs. The fly soon after grew weary, walked carelessly away, and was about to 

 take wing when she was captured. Her wings were much worn and ragged and she 

 appeared to have spent her supply of eggs before this attack. She lived but a few 

 hours in captivity. 



No eggs had been deposited on the larva during the attack, possibly because con- 

 ditions were not quite favorable. But the nature of the attempt explained the 

 finding of these dipterous eggs almost always on the inner surfaces of the prop-feet, 

 probably because this was the one vulnerable spot in the whole anatomy of the 

 caterpillar. 



The attack of the tachinid fly is often futile because the egg, after being placed on 

 the body of the larva, is likely to be shed with the next molted skin, before hatch- 

 ing. The question may also be raised whether the newly hatched tachinid larva 

 is always able to penetrate the tough, leathery body of its host. In almost every 

 case the eggs that were found had been placed within the crescent of hooks that 

 fringe the outer rim of the prop-feet, and were thus on the fleshy pad or sole of these 

 feet. In some instances, where the Hemileuca caterpillar, with the eggs placed on 

 the feet as described, was placed in captivity and the parasite reared, it was proved 

 that the young of the fly had in some manner been able to effect an entrance to the 

 body of the host. Eggs laid upon the sides of the thorax did not, as far as known, 

 injure the caterpillar upon which they were placed. , 



August 17, 1909, a full-grown Hemileuca larva, bearing two tachinid eggs in a 

 fold of the skin near the anterior pair of prop-legs, was taken in the vicinity of 

 Koehler, N. Mex. This infested individual was placed in a small box, where a week 

 later it was found in the act of making its crude cocoon. It pupated and five weeks 

 later a perfect moth appeared. This attack,- made under the most favorable cir- 

 cumstances, proved wholly abortive. 



It is possible, of course, that these flies choose the prop-feet as points of attack 

 because the body above is too well defended by its forest of needle-like spines to 

 permit of successful approach. Even if this is the case, it is a little strange that 

 eggs are not found elsewhere below than on the prop-feet, that are accessible to the 

 fly only when the caterpillar steps. 



Not infrequently, during August, molted skins were found to which were attached 

 1, 2, and even 3 tachinid eggs, generally fresh. In one case an egg was found on a 

 freshly cast skin, and one on the newly emerged larva that was standing close by, 

 showing that the fly had remained on duty during the operation of molting. 



