128 OUR HOUSEHOLD INSECTS 



nearly equal development of the three segments that 

 constitute the thorax; and yet a fourth is supplied by 

 the condition of the mouth organs. To examine these, 

 a little care is requisite. The head can scarcely be seen 

 at all from above, under ordinary circumstances, partly 

 because it is bent underneath at an acute angle to the 

 body, and partly because the greater portion of what 

 would otherwise be visible is concealed by the projecting 

 front of the semicircular shield which forms the roof of 

 the anterior segment of the thorax. If, however, the 

 insect be fixed on its back by a couple of pins thrust 

 through the sides of its thorax, and the head then lifted 

 up and turned back with the point of a pin, and retained 

 in that position by means of two pins crossing one 

 another, the mouth parts will be disclosed and may be 

 examined in detail with a hand lens. 



They consist of the usual parts. Taking them in the 

 order in which they are now placed, we find, first, the 

 labium, centrally situated, and clos- 

 ing the aperture of the mouth 

 below. By a little manipulation 

 with the point of a pin it may easily 

 be removed entire (Fig. 37), and if 

 then mounted in glycerine, may be 

 examined either with a hand-lens or 

 the compound microscope. It con- 

 sists of two basal transverse plates, 

 the larger one (submentum) behind, 

 and the smaller (mentum) in front. To 

 FIG. 37 .-Labiumof cock- the front of the mentum are attached 



roach, sm, submen- 

 tum ; m, mentum; i i n the centre two separate organs, 



ligula;p, la Dial palpi. f L 



called collectively the ligula ; each 

 is divided longitudinally into two divisions. Outside 

 these two organs are a pair, one on each side, of 



