METAZOA INSECTA. 463 



that at some remote time the food supply of these cater- 

 pillars failed and that gradually the habit was acquired 

 of moving in companies toward fresh grass lands upon 

 which to feed. Now the habit has become fixed in the 

 organization and is inherited. In this way the army 

 caterpillar has overcome conditions that would be adverse 

 to its own preservation. 



Suddenly a field that has been swarming with these 

 caterpillars is wholly free from them. This is owing to 

 the fact that they have descended into the ground where 

 their pupal life is spent. 



Erebus sfrex (No. 1175) is one of the largest and 

 handsomest members of the Noctuidae. 



The extremely interesting fact has been proved by 

 Gentry 1 that a normal cocoon-builder under certain con- 

 ditions may pass into the pupal stage as a chrysalis. 

 While the majority of the larvae of the Noctuid Acronycta 

 oblinita went through their transformations in the normal 

 manner, at least three without the slightest attempt at 

 cocoon-'making lay upon the soil and after a period of 

 five days entered the chrysalis state. Facts like this sug- 

 gest the origin and the evolution of the chrysalis-produc- 

 ing butterflies from the cocoon-making moths. 



The tussock moths or Lymantriidae have become well 

 known of late years through their representative the 

 gypsy moth, Porthetria ( = Ocneria) dispar Linn. (No. 

 1176; PI. 1177, figs. 1-6; Nos. 1178, 1179, large speci- 

 mens of the male and female). A cluster of eggs covered 

 by yellow hairs from the body of the female is seen at the 

 left in No. 1176 and in PI. 1177, fig. i, and a few eggs, 

 enlarged, in fig. 2. These eggs are laid in July, August, 

 and September, on the bark of trees, and the moth hiber- 

 nates in the egg stage. The following spring the cater- 

 pillars (No. 1176; PL 1177, fig. 3) are hatched and the 

 length of larval life probably averages ten weeks. If the 



iproc. Acad. Nat. Sci. Phila., 1875, p. 25. 



