PHYLUM ARTHROPODA. INSECTS 



257 



slowly devour and finally kill their host be- 

 fore it becomes an adult. Some parasitic in- 

 sects furnish examples of what is known as 

 polyembryony; each of their eggs produces 

 not one but many larvae, as many as 395 

 having been reported from a single egg. 

 Sometimes parasitic insects parasitize other 

 parasitic insects and these in turn may be 

 parasitized, and so on. Thus we have pri- 

 mary, secondary, tertiary, and quaternary 

 parasites, a condition known as hyper- 

 parasitism. There is certainly truth in the 

 following humorous lines: 



Big fleas have little fleas 

 Upon their backs to bite 'em 

 And little fleas have lesser fleas, 

 And so ad infinitum. 



Many insects are of the scavenger type, 

 and vast quantities of dead animal and 

 vegetable materials are eaten by them, thus 

 preventing decay and obnoxious odors. Blow 

 flies are especially effective, since they lay 

 enormous numbers of eggs, and the larvae 

 that hatch from them are extremely vora- 

 cious. As Linnaeus remarked, a fly can devour 

 the carcass of a horse more quickly than a 



Egg 



OJI 



Pupa 



Adult 



y 



Larva 



Figure 156. Life cycle of the housefly (Muscd domestica). This fly, like insects in general, has 

 a tremendous reproducti\-e potential. It has been said that if this potential were unchecked, the 

 descendants of a pair of flies in 10 years would weigh more than the earth. 



lion can. Water-scavenger beetles, burying 

 beetles, and dung beetles (Fig. 149), in- 

 cluding the sacred scarab of the Egyptians 

 are all scavengers. 



A study of the effects of certain maggots 

 as an aid in the healing of wounds has led 

 from maggot therapy to chemical therapy in 

 which synthetic allantoin is used directly in 

 treatment. 



Insects harmful to plants 



Insect pests, according to estimates by the 

 United States Department of Agriculture, 

 do about $4,000,000,000 damage annually to 

 farm crops, forests, stored foodstuffs, and 

 domesticated animals. Some are native pests, 

 such as the potato beetle (Fig. 157), Rocky 

 Mountain locust, and army worm; others 



