30 INSECTS AND MAN 



a series of changes is known as complete metamorphosis, 

 and this always occurs in the Coleoptera, Lepidoptera, 

 Hymenoptera, and Diptera. 



Now let us examine the common cockroach, popularly 

 but erroneously called the black beetle. Needless to say the 

 larvae arise from eggs, but, unlike those of the house fly, 

 they bear a very considerable resemblance to their parents. 

 As the life-history of the cockroach is fully described else- 

 where, there is no need to do more than mention here that 

 the larva undergoes several moults, and that, at each one, 

 it becomes larger and darker in colour, in fact more and 

 more like the adult. When the penultimate moult is reached, 

 the insect assumes the rudiments of wings and is then known 

 as a nymph ; the final moult produces the adult cockroach. 

 In such a life-cycle there is no quiescent or pupal stage, 

 and metamorphosis is accordingly said to be incomplete, and 

 it is the rule in the Orihoptera, Neuroptera, Rhynckota, 

 Thysanoptera. In the Aptera a third variety of life-cycle 

 occurs ; the young are similar to the adults, except in size, 

 and attain maturity by a succession of moults, in other 

 words, there is no metamorphosis. The life-cycles of all 

 insects are not always quite so straightforward as in the 

 selected examples ; sometimes, for instance, the larvae emerge 

 within the body of the parent and are brought into the 

 world in an active state ; this is the case, on occasion, in 

 the sheep bot fly, which for this reason is said to be vivipa- 

 rous. The length of time required for the completion of a 

 life-cycle, whether metamorphosis be complete or incomplete, 

 varies considerably in different insects; in some species 

 the time may be reckoned in hours, whilst, in the periodical 

 cicada, metamorphosis extends over a period of seventeen 

 years. External conditions such as food supply and tem- 

 perature exert a powerful influence on the duration of the 

 various stages; too high or too low a temperature will 

 influence the egg and pupal stages ; a scarcity of food may 

 also prolong the larvae stage. In the case of the ticks, 



