﻿ON ECONOMIC ENTOMOLOGY. 349 



Great humidity also quite unexpectedly influences the organism of insects. 

 It is well known that the spun products of larvae have a certain practical 

 importance in that they provide greater protection for the larvae and nymphae. 

 Bataillon 1-3 and myself 6 have experimented on the spinning of insect larvae in a 

 humid atmosphere. At various times I have ascertained that in such an 

 atmosphere larvae, while transforming into pupae, abstain from spinning. It is 

 thus possible to obtain nude pupae of species which normally produce pupae 

 enclosed in a spun cocoon. I found further that if caterpillars of Pieris brassicae 

 are placed on linen thoroughly saturated with water when the parasitic larvae of 

 Microg aster glomeratus are just emerging from them, the larvae do not spin their 

 cocoons, whereas under normal conditions they do so at once. Bataillon attributes 

 these results to the fact that transformation can only take place under diminished 

 osmotic pressure, to attain which the caterpillar or larva must rid its organism 

 first of the silk fluid, then of the contents of its alimentary system, including all 

 excreta, but finds itself impeded therein by humidity. 



Bataillon 1 ' 3 and myself 10 have also enquired into the influence of internal 

 respiration on transformation ; whereas Bataillon holds that the transformation of 

 larvae is brought about by an accumulation of carbonic acid in the organism, I 

 have endeavoured to prove that the oxidising enzymes are the main factor in 

 bringing about the transformation. The indispensable effect of oxygen may be 

 gathered from the observation that if adult larvae of flies are placed in a high 

 cylindrical glass filled with earth, those which are on the surface will first undergo 

 transformation, while of those in the soil the lowest are most retarded. On the 

 other hand, if adult larvae of flies are put into small glass tubes from which all 

 humidity is eliminated by using calcium chloride, they do not undergo any 

 transformation at all ; the same result being obtained if the larvae are placed in 

 an atmosphere charged with hydrocyanic acid. With Lepidoptera the latter 

 treatment produced also incomplete, soft-skinned, colourless pupae with only very 

 short wings. The influence of the hydrocyanic acid gas consists in the reduction 

 of the oxidising capacity of the tissues {see below). 



Whosoever has bred insects will have remarked that certain species quit the 

 egg or pupa at certain definite hours of the day. Occasionally reference is made 

 to this fact, not only as regards insects, but also as applicable to other animals. 



Recently I have closely watched for several weeks the spinning of caterpillars 

 of Lasiocampa quercus. This caterpillar, like that of Eriogaster lanestris 15 , forms 

 its cocoon by spinning a fine, closely woven tissue which is saturated from inside 

 by aid of the mouth, the caterpillar utilising therefor fluid, cream-coloured excreta 

 extruded from the anus. When I opened in the morning, about 8 a.m., the tin 

 boxes in which the caterpillars were kept, I regularly found silk cocoons. No 

 caterpillars started spinning in the afternoon. Only after 2 and up to 4 p.m. the 

 caterpillars set to work saturating the cocoon in the above manner. The 

 backward ones completed their work only between 5 and 6 p.m. The cocoon, 

 when quite dry, was incrusted with a hard mass. 



This peculiar behaviour reminds me of the so-called " bird clock " or time of the 

 day at which certain birds announce their waking up by emitting certain sounds ; 

 or of the " floral clock " of Linne which indicates the opening and closing of the 



