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CHAPTER II. 



; : PROTECTION AGAINST INJURIOUS INSECTS. 



Life-History of Insects. Insects belong to the Hexapoda ("six-footed") 

 class of animals having jointed feet (Arthropoda). Their bodies .consist of 

 three main sections (1) head, containing organs of sense ; (2) thorax, to 

 which six legs (three pairs), and generally also two pairs or one pair of 

 wings, are attached ; and (3) abdomen, containing organs of reproduction ; 

 and they pass through one or more of various stages of development 

 (Metamorphoses], each distinguishable from the preceding and the suc- 

 ceeding stages, namely (1) Ovum or egg, (2) Larva or grub, (3) Pupa or 

 chrysalis, and (4) Imago or mature adult insect. When all these four 

 stages are well defined, an insect is said to have a complete metamorphosis ; 

 but with many insects there is merely an incomplete metamorphosis with 

 no distinct pupal stage, because the larva gradually becomes transformed 

 into the imago, the pseudo-pupa being then known as a nymph, and the 

 transformation of a nym/>A-pupa into the perfect insect taking plac by 

 the already formed wings being liberated at the last moult or change of 

 skin ; while some wingless primitive insects (Aptera) undergo no 'meta- 

 morphosis at all. The Ova or eggs vary greatly in size, shape, and colour. 

 Eggs are laid singly or in clusters on different parts of trees, and some- 

 times lie naked and unprotected, or are protected within the bark or 

 by some special covering. The Larva usually hatches out in the course of 

 a few weeks, but often hibernates within the shell, and only emerges in the 

 following spring. The larva} of most beetles are called grubs ; the 1 6-footed 

 larvic of butterflies and moths (those of spanners have only 10, and a few 

 mining-moths have none) are called caterpillars ; the 18 - to 22 - footed 

 larvte of sawflies have tail-like extremities and are called tailed-cater- 

 pillars ; while the larvie of flies, which have neither feet nor any complex 

 structure of the head, are called maggots. The larva) of many beetles 

 have 3 pairs of legs on the first three (thoracic) segments after the head ; 

 the caterpillars of moths and butterflies have these also, and in additiou 



