182 THE PROTECTION OF WOODLANDS. 



from 2 to 5 pairs of prolcgs or clasping feet attached to the abdominal 

 segments, the last pair of which (claspers or anal prolcgs) are on the 

 terminal segment. As it gradually grows in size, the larva moults its 

 skin several times before pupation. The pupa sometimes lies unprotected 

 on the ground under moss and dead foliage, or in fissures or under bark- 

 scales, and at other times it is enclosed within a woven cocoon (often of 

 large size for some spinning-moths), while with flies the last larval skin 

 forms a protective covering. The longest stage of development is that 

 in which the insect hibernates ; and this is very often the larval stage, 

 although many beetles hibernate as adults. The egg and the pupal stages 

 usually last only from two to four weeks, except with insects which 

 hibernate thus. 



As soon as the adults appear, they usually pair at once and reproduce 

 themselves, the male generally dying soon after pairing, except in the 

 case of beetles, which often hibernate, and of bees which live for four or 

 five years. 



The Generation of any insect, or the complete cycle from egg to egg, 

 varies greatly, being multiple in plant-lice and ichneumon-flies, which 

 produce several generations in a year ; double in some bark-beetles and 

 sawflies, which produce two generations in each year ; single, simple, or 

 annual with most butterflies and moths, which yearly produce one 

 generation ; biennial or two - yearly in wood-wasps, the Pine resin-gall 

 tortrix, and many longicorn beetles ; and plurennial in the cockchafer, 

 which takes at least three and usually four years to complete its generation. 

 Occasionally there are two generations in three years (as in Bostriclms 

 bidens), but this is unusual. 



Insects with complete metamorphosis feed only as larvoo and adults, 

 though only in exceptional cases (e.g., Pine-weevil) doing damage as 

 adults ; but among insects with incomplete metamorphosis the nymph 

 also feeds. In both groups the larva) are often very destructive. 



Any attempt to group injurious insects either with regard to the trees 

 they attack or to the age of the crops generally attacked (seedlings and 

 young plants, thickets, pole- woods, and high woods) is unsatisfactory, as 

 many insects are dangerous at all stages of tree-growth, and feed more 

 or less indiscriminately on different kinds of trees when once they increase 

 in unusual numbers. But it may be remarked that most weevils, some 

 leaf-rollers, and cockchafer-grubs usually attack seedling growth and 

 young thickets, and that pole-woods and older crops are mostly attacked 

 by moths, and then by bark- and cambial-beetles when the trees grow 

 sickly. The caterpillars of the Pine owlet-moth and the Pine span- 

 worm always attack pole-woods before migrating to older crops on their 

 numbers increasing largely. With regard) to the parts of the tree chiefly 



