g INSECTS. PHYLUM ARTHROPODA 



skin splits and the imago emerges. Pupation, like ecdysis, seems 

 to be generally under the control of hormones. An imagmal 

 hormone, secreted in a part of the brain called the pars mter- 

 cerobralis, activates the thoracic gland (cells of the fat-body of 

 the pro- and meso-thorax) to produce another hormone which 

 stinuilates epidermal cell-division and the secretion of a new 

 cuticle ; it therefore induces moulting. In most insects the 

 thoracic glands atrophy in the adult and so moulting does not 

 continue. A third hormone, sometimes called the juvenility 

 factor, is secreted in the corpora allata, a part of the nervous 

 system near the heart. It stimulates the growth of the nymph, 

 but suppresses imaginal characters; metamorphosis can only 

 occur when production of the juvenility factor is reduced. Whether 

 this is gradual, or in two well-marked stages, determines the 

 absence or presence of a pupa. 



The instar before the pupa is often slightly specialised, and 

 is known as the prepupa. The rudiments of the limbs and wings, 

 which in the larva are held in sacs of the body-wall, become 

 everted, and the animal's behaviour changes. It is the prepupa 

 of butterflies which seeks shelter and spins the cocoon or mat. 



CLASSIFICATION 



The classification of insects is based primarily on the presence 

 or absence of wings, on the way in which these develop, and on 

 the degree of metamorphosis. Some of the wingless insects, 

 however, resemble so closely those which possess wings that they 

 are generally considered to have lost wings which their ancestors 

 once possessed, and are grouped with the winged orders. It is 

 for this reason that the names of the groups, derived from the 

 condition of the wings, are sometimes unfortunate ; by no means 

 all wingless insects are Apterygota, although that word means 

 ' creatures without wings '. It is very common now to use the 

 terms descriptive of the type of metamorphosis (Ametabola, etc.) 

 as alternative taxonomic proper names to those ending in 

 -pterygota, which refer to the wings. This introduces a second 

 set of illogicahties, and it is simplest to use the terms ending in 

 -metabola only as descriptions of the actual degree of meta- 

 morphosis undergone. 



The insects are divided into two classes, or sometimes the 

 two subclasses of the second class are raised to the rank of classes. 



