INSECTS, HARMFUL AND OTHERWISE 229 



alis, but they do not develop into butterflies or moths. The 

 caterpillars attacked by parasites generally are restless, and 

 show evidence of discomfort or pain. Often we find chrys- 

 alises, of the cabbage caterpillar, for example, with little 

 holes, or punctures, in the case. They had parasites which 

 came out through the holes. By these parasitic insects 

 many injurious caterpillars are kept more or less in check. 

 Biological relations in nature are very complex. Each 

 species must have nourishment, and this must constitute 

 some other form of life, plant, or animal. In this way a fair 

 balance is kept between the different kinds of life, one acting 

 as a check upon the other. Mr. F. S. Mathews' quotes an 

 appropriate doggerel to express this idea in another way : 



Little fleas have lesser fleas upon their backs to bite 'em, y 



And these fleas have lesser fleas ad infinitum. 



Great fleas have greater fleas upon their backs to go on, 



And these fleas have greater fleas, and greater fleas, and so on. 



Some Good or Harmless Insects 



The honey-bee has already been described on page 205. 

 Another domesticated insect, which is always interesting to 

 children, is the Silk-moth. Silk-worms are reared artificially 

 in a number of countries of Europe and Asia. The cater- 

 pillars are carefully tended and fed with mulberry leaves, and 

 when they have reached the pupa stage, they spin around 

 them a cocoon of a fine strong fibre, the silk which we use. 



The fibre is derived from a gland in the body of the cater- 

 pillar, and issues from a spinneret on the lower side near the 

 mouth as a liquid, which, however, solidifies in the air. 



In the silk culture the cocoons are collected and the pupae 



1 " Familiar Life in Field and Forest." 



