332 A BOOK OF INSECTS 



order ; while it is believed that the Dibrachys itself may 

 sometimes occupy the third place, in which case the 

 Asecodes might become a parasite four times removed 

 from the original host. In all cases of this kind the odd 

 numbers are beneficial from the economic standpoint, the 

 even numbers being injurious ; but it is clear that the 

 relationship of one insect to another may be extremely 

 intricate, calling for much careful study before it can be 

 rightly interpreted. 



Proof is not wanting that the growth of noxious 

 weeds may be checked by insects, while we have already 

 seen that many of these creatures play an important part 

 as scavengers in the economy of nature. A few kinds of 

 insects are directly serviceable to mankind. Certain 

 species constitute an important food item among savage 

 and semi-civilised races. Thus, locusts are eaten in great 

 quantities in Africa, termites in Africa and Australia, 

 while a species of water-bug and its eggs are highly 

 relished by the natives of Mexico. Many other kinds 

 of insects are eaten, but they are for the most part 

 regarded as luxuries rather than as staple articles of diet. 

 Several species of soft-skinned beetles, whose blood con- 

 tains large quantities of cantharidine, are used in allopathic 

 medicine — the European blister beetle (Lytta vesicatoria), 

 often called the " Spanish Fly," being the most important. 

 The pharmacopoeia of the homeopaths includes a number 

 of insects (e.g. the hive-bee, the common cockroach, and 

 the seven-spot ladybird), all of which, so the writer is 

 assured by an expert, have proved useful in alleviating 

 certain of the ills to which flesh is heir. The chief 

 commercial products of insects are silk, honey, and wax. 

 Most of our silk is derived from the cocoons spun by 

 the caterpillars (" silkworms ") of Bombyx trior i, which 

 are extensively cultivated in Southern Europe and the 



