176 INSECT TRANSFORMATION 



reproductive organs whose ducts open far back on the abdo- 

 men. Insects breathe by means of branching air-tubes 

 which open by paired spiracles on the sides of most of the body- 

 segments. Like other Arthropoda, insects produce large 

 eggs with a considerable quantity of yolk ; these are generally 

 laid by the female and subsequently hatched, but in some cases 

 they undergo development within the body of the mother, 

 who thus gives birth to active young. When born or hatched, 

 young insects are always without wings ; in other respects 

 they may resemble their parents closely or they may differ 

 from them very markedly in outward appearance. During 

 the period of growth, which is necessarily accompanied by a 

 series of moults, the wings are developed, the general aspect of 

 the body remaining much the same throughout the process, 

 or undergoing a marked transformation or metamorphosis. 



The Class of the Insecta is divided into three sub-classes, 

 which are distinguished from each other by the nature of the 

 wing-growth. 



Sub-ClaSS I. APTERYGOTA 



These are wingless Insects in which the wingless condition 

 is believed to be primitive. The mandibles are more like 

 those of the Crustacea than those of other insects, and the 

 maxillulae are well developed. Abdominal limbs are present 

 in the adult. Transformation is slight or absent. 



Three orders are included in the Apterygota. 



Order i. Thysanura 



The Thysanura or bristle-tails are wingless insects with 

 elongate feelers, an abdomen with ten segments, the hindmost 

 of which bears a pair of prominent limbs (usually jointed 

 cerci), while short un jointed paired limbs (stylets) are present 

 on other abdominal segments (from the second to the ninth, 

 inclusive, or fewer). There is a well-developed system of air- 

 tubes. The eighth and ninth abdominal segments carry 

 prominent reproductive processes (gonapophyses). 



Newly-hatched Thysanura may differ from the adults in 



