42 INSECTS AND HUMAN WELFARE 



species of insects were phytophagous in habits, feeding di- 

 rectly upon the tissues of various plants. This proportion is 

 probably not far amiss as Weiss has recently tabulated the 

 food habits of the 10,000 insects of New Jersey enumerated 

 in Smith's list. He finds (Fig. 18) that 48,2 per cent of these 

 are phytophagous; that 16 per cent are predatory, feeding 

 mainly on other insects; that 17.3 per cent are saprophagous, 

 living as scavengers; that 12 per cent are entomophagous 

 parasites, developing in the bodies of other living insects; 

 and that of the small residuum of 6.5 per cent, 2.4 per cent are 

 epizoic parasites of vertebrates. Only 2.1 per cent appear to 

 be of no human importance in so far as we can judge at the 

 present time. 



Of course, the great majority of phytophagous insects af- 

 fect plants never or rarely utilized by man and bear no direct 

 relation to food production. However, agricultural plants 

 show no immunity to insect attack, but quite decidedly the 

 opposite condition prevails, that they are more susceptible; 

 and this is undoubtedly due to several causes which I shall 

 attempt to enumerate. 



The species of plants suitable for cultivation as sources of 

 food, are naturally those which produce more than average 

 amounts of food material in some part of the plant body. 

 This may develop in the roots, as in carrots, in the stem as in 

 sugar cane, in the foliage as in spinach, in the seeds as in 

 beans, in the tissue enveloping the seeds as in many fruits, 

 or more rarely in special organs. On this account alone, such 

 plants are unusually acceptable to insects as they are to our- 

 selves. Another reason for the great susceptibility of agri- 

 cultural plants to insect injury depends upon the removal of 

 certain barriers to insect multiplication which follow neces- 

 sarily as a result of all agricultural development. Under 

 natural conditions, plants almost always occur in complex 

 associations of numerous species, of which one or several may 

 be noticeably dominant, and many of these are capable of 

 assuming a temporary dominance when for any reason the 

 more abundant species decline. Such a stable, gradually 



