INSECT ECOLOGY 375 



become fixed, whereby diverse plants are utilized by oligophagous 

 species." 



The selection of food by means of its odor is simply a case of positive 

 chemotropism (see page 302), a blind reaction to a chemical substance. 

 Pomace flies, which feed on and lay their eggs in fermenting fruits, are 

 positively chemotropic to weak percentages of certain alcohols and 

 acetic acid, which are products of fermentation. House flies are stimu- 

 lated to oviposition by ammonium carbonate, with its odor like that of 

 manure. The cabbage butterfly is induced to lay eggs by mustard 

 oils, which occur naturally in its usual food plants, Cruciferae. 



Growth. Other things equal, the length of the larval stage depends 

 upon the kind, condition, and amount of food. With the house fly, it 

 is primarily temperature and moisture that determine the rate of devel- 

 opment; but with an average temperature of about 21 C., the maggots 

 develop in horse manure in fourteen to twenty days, and on a diet of 

 bananas, in twenty-seven days. At the same temperature, the rate of 

 development is directly proportional to the condition of the food as 

 regards moisture. Dry conditions may retard development five or six 

 weeks, tend to produce flies of subnormal size, or may be fatal. (C. G. 

 Hewitt.) 



If the mother insect lays her eggs in a considerable supply of food 

 substance, as happens usually in the case of the house fly, pomace fly, 

 :arrion beetles, dung beetles, and many other insects, the development 

 >f the larva is assured, so far as the amount of food is concerned. The 

 [uantity of food present becomes important, however, for insects that 

 ire restricted to one kind of food plant, or to a food of low nutritive 

 e. The nutritive content of wood is small, and wood-boring cater- 

 )illars and grubs frequently require long periods for their growth, even 

 ;veral years (Cerambycidae) ; though less time is needed if the larvae, 

 like those of the peach tree borers and the flat-headed apple borer, 

 feed largely on the inner bark, which is more nutritive than the 

 *ood. 



Size. Under-nourished larvae produce small adults, as might be 

 expected. The size of boll weevils depends upon the abundance of the 

 food supply and also upon the nature of the food. The smallest weevils 

 develop from squares which are very small, and which fall very soon 

 after the egg is deposited; the largest, from bolls which grow to maturity. 

 In bolls the food supply is most abundant, and the period of larval 

 development is several times as long as it is in squares. (Hunter and 

 Pierce.) 



