16 



THE ARMY-WORM. 



their presence was discovered. Some fields, that looked very 

 promising in the distance proved on closer examination to 

 be very badly injured by having the berries exposed to the 

 drying influence of sun and wind which shrunk them badly. 

 A closer inspection of all fields showed at once that the 

 worms had been at work for some time; in fact they had 

 eaten up all the foliage and the ground was thickly covered 

 w^ith bits of leaves and the frass or droppings. In most of 

 the fields the caterpillars were not forced to migrate on ac- 

 count of lack of food, as enough of it was still left to enable 

 the almost full-grown worms to mature. It seems a sort of 

 instinct that prompts them to move away from such fields 

 and to move toward new pastures. The armies thus formed 

 are not composed even of social insects such as the tent- 

 caterpillar, but are more like a mass of individuals routed 

 by a common enemy. All worms run away and do so re- 

 gardless of their neighbors. When 

 we look into the matter a little 

 more carefully we find, as a gener- 

 al rule, that it is not simply hun- 

 ger that prompts them to wander, 

 but that they are frequently har- 

 assed by numbers of parasitic in- 

 sects. Out of ten worms picked 

 least six or more that harbor 

 maggots of flies or of small 

 parasitic wasps. In one case fifty worms were picked up at 

 random and forty of them showed the peculiar china-white 

 and glossy eggs of a fly shown in fig. 8. The adult tachina- 

 fly {E.roi'lxfa leuanuce Walsh.), a little larger than a common 

 house-fly, could also be observed in large numbers near the 

 army-worms. It would dart toward an intended victim 

 and notwithstanding the fact that this always tried to es- 

 cape, the fly would succeed in fastening one or more eggs 

 upon its neck. The maggots hatching from such eggs pen- 

 etrate into the worm and there feast upon the material 

 stored up to produce the future wings and other organs of 

 the adult army-worm or moth. Other parasites were also 

 very numerous, but their methods can not as readily be ob- 



Fig. S. — Tachina-fly of armj' 

 worm. Enlarged. After Walsh 



up 

 in 



we may find at 

 their inside the 



