THEIR RELATION TO WEATHER 149 



species of plant lice are so heavily parasitized that 

 even when they get a good start early in the year, 

 their enemies usually overhaul them before they have 

 destroyed or even severely injured their host plant; 

 nevertheless I am ready to allow my statement to stand 

 as an expression of general conditions, applicable to 

 the ordinary run of species. 



I have already referred to the fact that caterpillars 

 are subject to disease and the check to certain species 

 is, I believe, much greater than is generally recognized. 

 On that point I made a very interesting study on a 

 large series of light cocoons of the Cecropia moth, 

 finding that more than two thirds of all the caterpillars 

 died of some trouble other than parasites after the 

 cocoons had been completed, but before the change 

 to pupa had taken place. Others as well as I have 

 observed entire broods of caterpillars dying, and one 

 of the characteristic attitudes of such diseased cater- 

 pillars is that the twig or leaf is clasped by the pro- 

 legs along the sides of the body, while the portions 

 anterior and posterior to this hang limp and lifeless. 

 Eventually the whole thing dries and shrivels up almost 

 to a skin. This sort of condition I observed in Massa- 

 chusetts in 1907, in territory infested by gypsy and 

 brown-tail moths; fully 50 per cent, of all the larvae 

 seen showing evidences of disease: and that condition 

 existed to a still greater extent in 1906, when a large 

 proportion of the caterpillars of the brown-tail moth 

 were wiped out of existence. 



This is eminently one of those cases where an ear- 

 nest effort should be made to use the disease-producing 

 organisms to check increase and spread, and the study 

 of the subject has been actually taken up. It is quite 

 probable that it will be found that the disease has its 

 limitations, and that it requires certain climatic con- 



