GARDEN PESTS IN, NEW ZEALAND 



CHAPTER III. 



Structure of Insects, 



ALTHOUGH insects present a great variety of forms, they never- 

 theless agree in general features; thus by studying the structure 

 of some generalised species, which will give a broad idea of the main 

 characteristics, one is enabled to recognise different structural modifica- 

 tions assumed by various species. For this purpose a weta, grasshopper, 

 or cockroach may be taken as a type. 



Just as in the case of the crayfish, so the body of an inisect is 

 completely covered and protected by a continuous "shell/' very solid 

 in some insects, more or less pliable in others, but even in the most 

 delicate forms tending to become rigid and brittle after death. This 

 shell acts as a skeleton and as a very effective armour-plating, protecting 

 and supporting the soft body within. Unlike the shell of the crayfish, 

 which is mainly calcareous, that of insects consists of a horny substance 

 called cliitin, secreted by the underlying skin, and constitutes what is 

 known as a cuticle. It is due to this horny cuticle or shell that the 

 form and colour of most insects are preserved after death, though the 

 enclosed body tissues decay unless preserved in some suitable medium. 



The cuticle, though forming a complete covering, does not enclose 

 the body in an inflexible shell ; flexibility is allowed by the cuticle being 

 formed of a segmented series of strongiy-chitinised sections alternating 

 with skin-like, feebly-chitinised, and very elastic sections; this arrange- 

 ment gives freedom of movement to the enclosed body, as is readily seen 

 in the movements of a caterpillar. 



There are three distinctly separated divisions of the insect body 

 head, thorax, and abdomen each consisting of a varying number of 

 segments (Fig. 3). The head segments are so closely fused as to be 

 practically untraceable, the cuticle forming a rigid capsule; the thorax, 

 to which the head is attached, carries the wings (when present) and 

 the legs, and consists of three segments; posterior to the thorax is the 

 abdomen, comprised of several segments, which show the typical 

 segmentation of insects better than any other part of the body. 



The head capsule is more or less freely movable on the thorax, and 

 bears certain sensory organs, together with the mouth appendages. The 

 sensory organs are the eyes and the feelers, or antennae. On each side 

 is a compound eye of varying size, according to 1 the insect; each eye 

 consists of a variable number (from a comparative few to several 

 thousand) of microscopic, hexagonal lenses, each of which records a 

 separate image; Between the compound eyes, on top of the head, are 

 three simple eyes in some insects, but in others one or all of these may 

 be absent. Between the compound eyes on the front aspect of the head 

 is a pair of feelers, or antennae; they consist of a variable number of 



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