156 PROTECTION AGAINST INSECTS. 



development, for example, two years in the case of longicorn 

 beetles and Sirex, and three or four years for the cockchafer. 



Of the several stages of the insect, that in which it hiber- 

 nates lasts the longest, and is generally the larval stage. The 

 egg and pupal stages usually last for about two to four weeks 

 unless they happen to be the hibernating stage, and eggs laid 

 after Midsummer do not generally hatch out till the succeeding 

 year. As a rule, the imago stage is the least long-lived ; but 

 with beetles this is not the case, as the images very frequently 

 hibernate. Many Ilymenoptera are long-lived insects ; bees, 

 for example, live four or five years. 



F. Eiihl, of Zurich,* has observed the life-period of a 

 few beetles in their various stages : eggs, 5-44 days ; larvae, 

 47-1640 days (many longicorn larvae live longer) ; pupae, 8-39 

 days ; beetles, 3-60 days. As a rule, insects with fully 

 developed sexual organs live for a short time only, while 

 unsexual insects live longer. 



Many families of insects, for instance, bark-beetles, have 

 irregular broods ; the state of the weather, and the quantity and 

 quality of their food may cause the development of one, two, 

 or even three broods in a year. On the other hand, certain 

 circumstances, such as unfavourable weather, want of oppor- 

 tunity for pairing or for laying eggs, may not unfrequently 

 cause delay in a brood. Thus, for instance, a brood of the 

 pine sawfly (Lophyrus pini, L.) has been known to extend 

 over 1J to 2 years, instead of there being one or two broods in 

 the same year. 



2. Habitat. 



The habitat of insects varies according to their state of 

 development and the season of the year. They are sometimes 

 found on or under the surface of the ground, or on woody or 

 herbaceous plants. They generally hibernate under the soil- 

 covering^ or inside the bark of trees. As a rule, all insects 

 live near the material on which they feed, but many wander 

 far, sometimes against their will, as when they are blown into 

 the sea by storms of wind. They generally endeavour to return 

 to their former abode, as in the case of bees and ants. 



Examples of insects that have been imported to Europe 



* " Centralblatt fur das Ges. Forstwesen," 1888, p. 156. 



