232 



COCKROACHES. PHYLUM ARTHROPODA 



and at its sides a pair of minute structures, the superlinguse, 

 which may represent the paragnatha of the crayfish (p. 204) ; 

 (ii) the narrow gullet, lying in the neck ; (iii) the swollen crop ; 

 (iv) the proventriculus or gizzard, which has muscular walls 



F"iG. 167. — A diagram to show how the wings of an insect are lowered and raised 



in flight. 



.4 , The downstroke : the tergum (t) is raised, owing to being arched fore and aft by the contraction of 

 the longitudinal muscles (l.m.) ; this forces the wing {w.) down, pivoting over a point on the pleuron 

 (pL). B, the upstroke : the tergum is lowered by contraction of the dorso- ventral muscles (dv.m.) ; this 

 levers the wing up. 



and contains six hard, cuticular teeth and some pads covered with 

 bristles which form a strainer. Two diffuse labial or salivary 

 glands lie on each side of the crop, and between each pair lies a 



^' ling. 



mx.n. 



Fig. 168. — A semi-diagrammatic drawing of the head and thorax of a cockroach, 



dissected from the left side. 



cer.. Cerebral ganglion; cr., crop; fr.g.. frontal ganglion giz., gizzard hp.c. hepatic caeca l.v.n., left 

 visceral nerve leaving the brain ; Ibm., labium ; ling., hypopharynx ; Ibr., labrum ; m.g., mesenteron, 

 md., mandible ; mx.n., maxillary nerve ; nk., neck ; oes., oesophagus ; s.as., suboesophageal ganglion ; 

 sal.g., sahvary gland ; sal.r., sahvary receptacle ; th.i, th.2, th.^, segments of the thorax ; v.g., visceral 

 ganglion ; v.n., visceral nerve. 



salivary bladder or receptacle. The ducts of the two glands of 

 each side join ; the common ducts of the two sides then unite to 

 form a median tube, and this is joined by another median tube 

 formed by the union of the ducts of the receptacles. The final 

 opening is on the hypopharynx. The mid-gut or mesenteron, 

 lined by soft endoderm, is short and narrow and bears at its 



