308 Studies in Forestry [CHAP. xiv. 



The Life-history of Forest Insects. 



Insects (Insectd] are animals with jointed feet (Arthropodd], 

 which have a body consisting of three main sections, six legs 

 (three pairs), and usually also four or two wings (two pairs or 

 one), and which pass through various stages of growth. Most 

 insects undergo four stages of development or Metamorphoses, 

 each distinctly distinguishable from those which precede and 

 succeed it. These are in turn (i) Ovum or egg, (2) Larva, grub 

 or caterpillar, (3) Pupa or chrysalis, and (4) Imago or mature 

 insect. When all these separate stages are passed through an 

 insect is said to have a complete metamorphosis; whilst the 

 comparatively few instances (forming the orders Hemiptera and 

 Orthopterd] in which a distinct pupal stage is not apparent, 

 as the larva gradually becomes transformed into the imago, 

 are termed an incomplete metamorphosis. The former are called 

 metabolian (insecta metabold], and the latter ametabolian (insecta 

 ametabola\ whose pseudo-chrysalides are known as nymphae. 

 The metabolic class comprises about 95 % of all insects, and 

 the ametabolic only about 5 %. The Ova or eggs vary 

 greatly in shape, size, and colour. Ovideposition may take 

 place singly, or in clusters or nests on different parts of trees ; 

 they are sometimes protected by some sort of covering. The 

 Larva often makes its appearance from the egg within the 

 course of a few weeks ; but in many other cases it hibernates 

 within the shell, and only issues during the following spring. 

 Different kinds of larvae have special names. Those of most 

 beetles, partly six-footed, partly without feet, are called larvae, 

 except in the case of cockchafers (Melolontha), where they are 

 termed grubs ; the sixteen-footed larvae of butterflies and moths 

 those of Spanners (Geometridae) have only ten, and a few 

 Mining-moths (Tineidae] have none are called caterpillars \ 

 the eighteen- to twenty-two-footed larvae of Sawflies (Tenth- 

 redinidae) with their tail-like extremities are named tailed cater- 

 pillars ; and the feetless larvae of flies (Dipterd]^ which show 



