INJURIOUS AND USEFUL INSECTS 



2. (PRACTICAL.) THE EXTERNAL PARTS OF AN 

 INSECT (Cockroach) 



Wanted : fresh cockroaches. They can be instantly killed 

 by heat. Place the number required in a beaker, and dip the 

 outside of the beaker in boiling water, or put one or two into 

 a test-tube, and warm over a spirit-lamp. 



Distinguish the head, which bears the feelers and jaws; 

 the thorax, which bears the legs and wings ; and the abdo- 

 men, which does not bear any appendages for mastication or 

 locomotion. 



The body is made up of the following segments : — An un- 

 certain number, at least six, compose the head ; the thorax 

 consists of three segments, and the abdomen of at least ten. 

 A segment usually consists of a dorsal plate (tergum) and a 



ventral plate (sternum), to 

 which side-pieces (pleura) may 

 be added in some cases. In 

 typical segments a pair of appen- 

 dages is attached between the 

 pleura and the sternum. 



Observe the attachment of 

 the head. It is carried on a 

 rather long and slender neck, 

 and can be protruded or re- 

 tracted, while the lower or 

 mouth-end can be inclined 

 either forwards or backwards. 

 When the insect is at rest, the 

 head hangs nearly vertical, but 

 sloping a little backwards. Ob- 

 serve the long many-jointed 

 antennae or feelers, the com- 

 pound eyes on the sides of the head, and the jaws. A pair of 

 white spots, which are perhaps undeveloped simple eyes, will be 

 seen to the inner side of the base of the feelers. In front of the 

 jaws hangs down a short flap, the labrum. On each side of the 

 labrum is a pair of stout toothed jaws, the mandibles, which work 

 from side to side. Separate one of these with a pin to see how 

 it works. Examine the labrum and mandibles with a lens. 



Fig. 2. 

 Front of head of cockroach. 



