164 THE CANADIAN ENTOMOLOGISt. 



again obtained and probably will never do so. The first conditions, then, 

 for the presence of a species of Lepidoptera are those of warmth and 

 food — a minimum of cold to be supported, a sufficiently extended time of 

 warmth during which the insect can provide for its metamorphoses. The 

 summers running too short, will prevent the existence of species, even 

 where the food is abundant ; while a few kinds of Arctic butterflies seem 

 to take two years in which to perform their life changes, an acquired 

 habit with the other phenomena of hibernation. In this respect there is 

 a difference of hardiness between the species ; it is probable that the 

 average temperature is not of so much consequence as the point of its 

 absolute lowness at given times, exposure to which, in certain of its states, 

 the insect cannot survive. The same amount of cold might be innocuous 

 to the egg, which would kill the chrysalis. That food itself is not suffi- 

 cient for the presence of the species to which it is adapted, is a fact well 

 known to collectors. While there are a good many accidental causes to 

 account for this, in any one region, it is found also that the range of the 

 plant is not necessarily co-extensive with the range of the species feeding 

 upon it ; here the climate (humidity, degree of cold) is one of the 

 determinating causes. After warmth and food we must next consider 

 enemies, parasites, competing species. This is a vast field for observa- 

 tion. The number of ichneumon flies is apparently influenced by special 

 causes, so that in some years they do not destroy so many caterpillars ; in 

 shifting their ground the lepidopterous hosts sometimes evade their guests 

 for a season. The birds destroy yearly a large percentage of these 

 insects, but they, as well, are more or less plentiful from causes which are 

 independent of the supply of insect food. 



There is then to be considered the physical geography and the geology 

 of the country. What are called by Entomologists "Chalk insects," are 

 those species which inhabit by preference this formation, the geology of a 

 district influencing its flora, and this in turn its insects. Forests are also 

 protective to some considerable extent, .less from depredators and enemies 

 than from high winds, which tear our frail friends to pieces ; and from 

 sudden changes of temperature at an awkward moment, such as the 

 change of dress from the caterpillar to the chrysalis state, or the previous 

 and various steppings out of the larval skin, which, like getting out of 

 one's trousers, is always a risky undertaking. The forest itself may seem 

 to be bare of insects as compared with the open fields, where the broad 

 bits of color of a butterfly's wings come into quick notice. But, in 



