200 Annals Entomological Society of America [Vol. XIII, 



to them, so that they are much more destructive in the second 

 half of the insect life cycle than in the first; but in a holometa- 

 bolous insect with unlike larval and adult habits and habitats, 

 this period is divided into two unlike periods of disease or 

 infestation, and there is no second half to either of the two. 



The multiplication of seasonal generations in some species 

 is a consequence of a high degree of sensitiveness to temperature 

 and other developmental stimuli; and this may enable a species 

 to push its range into colder latitudes than would otherwise 

 be possible, giving it at the same time a capacity for multiplica- 

 tion in the milder latitudes far in excess of that of its single- 

 brooded competitors and enabling it to take prompt advantage 

 of seasonal conditions temporarily favorable and to rally 

 quickly from the effects of those temporarily injurious. I sur- 

 mise that the many-brooded species have, as a rule, had an 

 experience of frigid or semi-frigid life — that of a glacial period, 

 for example — during which variations towards a quick physio- 

 logical sensitiveness to heat stimuli have been selected for 

 survival. Entomologists are but just beginning to determine 

 accurately the so-called physiological zero or threshhold of 

 development of the several insect species, and have accumulated 

 as yet too little precise knowledge of the temperature at which 

 development begins and of the effects of differences of humidity 

 in shifting this zero up or down to enable us to base our surmises 

 on experimental evidence. There is an almost limitless field 

 for interesting investigation open to those who have command 

 of a good experimental equipment, and the entomologist who 

 first carries through a seasonal series of experiments on the 

 army-worm with its three annual generations in comparison 

 with one of the single-brooded noctuids, both kept together 

 under identical* conditions for a study of their differences of 

 reaction to ecological factors, will get some new and important 

 results. 



As I piece together, after a fashion, these few scraps and 

 fragments of observation, interpretation, and inference which 

 I am offering here, it seems to me that the general pattern 

 which they suggest is that of a wide-ranging, open-minded 

 survey of insect life as it is actually lived by these complex and 

 variable creatures, in constant interaction with the still more 

 complex and similarly variable system of objects and energies 

 which together constitute the insect's world — that we need to 



