244 ARTHROPODA [CH. 



gland opens to the exterior by means of a short muscular tube, 

 the ductus ejaculate rius, which has its orifice just below the 

 anus. The name of the gland is derived from its form ; it has a 

 thick stalk surrounded by a crown of branches. Fertilisation and 

 oviposition take place during the summer. 



Sixteen eggs are laid in each egg-capsule, and for some seven 

 or eight days, until the mother finds some warm and secluded 

 hiding-place to deposit her load, she carries about the capsule half- 

 protruding from the genital pouch. When the embryos in the eggs 

 are fully formed, which takes about twelve months, it is said that 

 they secrete some fluid, probably saliva, which dissolves the upper 

 part of the capsule and so permits of their escape. In Phyllodromia, 

 germanica the mother is said to take a part in freeing her offspring 

 from their temporary imprisonment. When they first appear they 

 are white with dark eyes, but the integument soon thickens and 

 darkens. They have no wings, but in other respects they resemble 

 their parents and thus there is no metamorphosis such as occurs 

 in the Butterflies and many other Insects. They run actively about, 

 devouring any starchy food they can find, and when in time they 

 grow too large for their coat of mail it splits and a soft cockroach 

 extricates itself therefrom. The integument soon hardens again. 

 This casting of the skin or ecdysis takes place seven times, and after 

 the seventh moult, when the insect is four years old, it is adult. 



The Insects are subdivided into orders, which are mainly based 

 (i) on the structure of the gnathites ; (ii) on the nature of the wings; 

 (iii) on the amount of metamorphosis which the life-history of the 

 Insect presents. A short account of each of these criteria is therefore 

 subjoined. 



The Mouth-Parts (Gnathites) of Insects. 



The mouth-parts of Insects can m almost all cases be resolved 

 into a pair of mandibles which never bear palps and two pairs of 

 maxillae which are usually provided with palps. There is present 

 in the embryo in addition a pair of vestigial appendages situated 

 behind the mouth. These in many insects coalesce to form the 

 tip of a spine-like structure termed the hypopharynx. In the 

 different orders of Insects and in different members of the orders 

 these mouth-appendages show many modifications and are put to a 

 very great variety of uses. One or other part may be suppressed and 

 disappear, others may coalesce, as is the case with the right and left 





