14 INJURIOUS AND USEFUL INSECTS 



position. The balsam should be just enough to fill neatly the 

 space beneath the cover. If too little has been used, more may 

 be added afterwards by touching with a glass rod dipped in 



balsam one of the angles 

 where the balsam recedes 

 from the edge of the cover. 

 Any small excess of balsam 

 can be removed when per- 

 fectly dry — i.e. after some 

 weeks or months. If there 

 is a considerable excess, re- 

 mount the piece. Do not 

 try to force the balsam to 

 fill a vacant space by pres- 

 sure. When the prepara- 

 tions indicated have been 

 mounted, examine them by 

 a low power of the micro- 

 scope, compare them with 

 the figures, and draw them. 

 The mouth-parts of the 

 cockroach, and of other 

 insects with biting jaws, 

 consist of a flap, the lab- 

 rum, which hangs down in 

 front of the mouth ; a pair 

 of stout mandibles, usually 

 adapted either for cutting 

 or crushing, or both ; a pair of maxillae ; and a second pair of 

 maxillae, which become more or less fused together, and con- 

 stitute the organ called labium. It is unfortunate that words 

 so similar as labium and labium should have been accepted, as 

 there is a tendency, even among experienced entomologists, to 

 mistake one for the other. Here is a diagram of the typical 

 mouth-parts, such as occur in the cockroach : 



Labrum 



First pair of jaws (mandibles). 



Second „ (maxillae). 



Third „ (labium, or second pair of maxillEe). 



The jaws of insects and other Arthropods are modified legs. 



F'g- 13.— Labium of cockroach (united 

 maxillae of the second pair). 6V«, sub- 

 mentum; the other letters as in fig. 12. 

 X 20. 



