XII] COCKROACH 233 



on the ventral surface a pair of legs and the two hindermost of 

 which bear on the dorsal surface a pair of wings. The abdomen 

 consists of a varying number of segments, ten being perhaps the 

 usual number but fewer often occur. The abdomen bears no 

 appendages except at the posterior end, where a pair of rod-like 

 outgrowths the anal cerci are often found. 



Owing to the similarity of Insects to one another and their great 

 number the study of them has become a very special branch of 

 Zoology, which is termed Entomology. The necessity of ex- 

 tremely detailed study is due to the same cause and a great number 

 of technical terms are in use for describing the numerous structures 

 which build up the body of the Insect. 



In this short book it will only be possible to indicate a few 

 points about the anatomy of Insects and we shall take as a type the 

 common cockroach because it is both a generalised form and not 

 too small for dissection. 



The common cockroach of the British kitchen is Stylopyga 

 orientalis, but a larger form, Periplaneta americana, is often met 

 with on ships and from them makes its way to the docks; it is also 

 often found in zoological gardens, etc. Phyllodromia germanica, 

 a small species, is becoming increasingly common in England. 



The whole body of the cockroach is covered by a chitinous 

 covering which varies in thickness, from the black hard head to the 

 thin whitish areas which exist at the joints and which permit 

 movement of the harder parts on one another. Except in the 

 head the segments of the body can be detected externally, and, as 

 in other members of the Arthropoda, the segmentation affects some 

 only of the internal structures, such as the heart, the tracheae, the 

 muscles and the nervous system, the other organs of the body not 

 being influenced by it. 



The head of a cockroach is a flattened structure placed at right 

 angles to the axis of the body. It is oval in outline, its upper edge 

 being considerably broader than the lower. It is loosely jointed to 

 the thorax by a neck which permits considerable movement (Fig. 97). 

 This neck enters the head near its upper edge and below it the 

 head hangs free. On the upper and outer edge of the head are 

 a pair of kidney- shaped, faceted eyes of a shining black colour, 

 on the inner curve of which the antennae or feelers have their 

 origin. These are long whip-like structures, often as long or longer 

 than the body; they are made up of many joints and during life 

 are in active movement, now stretched downward as if trying the 



