VI ANTENNATA— MOUTH PARTS 445 



Apterygota the Thysaniim possess 10 abdominal segments, and the 

 CoUembola a varying number, but always less than 10. In the Pterygota 

 the number of abdominal segments in the adult animals varies, and is 

 generally less than 10. This diminution is caused by the fusing of those 

 segments which are connected with the genital apparatus and lie in 

 front of the last, and secondly by the fusing of the anterior abdominal 

 segments (usually only the first) with the thorax. On the other hand 

 in a few insects {Macrolepidoptera, Diptera, and Rhyncliota) the last 

 (3d) thoracic segment is joined with the abdomen. 



B. The Limbs. 



The limbs of the Insecta consist of single rows of joints. We 

 distinguish the limbs of the head from those of the trunk. It is 

 certain that each trunk segment was originally provided with a pair of 

 limbs (as is now the case in Peripatus and the Mijriapoda). In the 

 Hexapoda, however, only the limbs of the 3 anterior trunk segments, 

 i.e. of the thorax, have been retained. 



1. The Limbs of the Head. 



There are, typically, 4 pairs of cephalic appendages, which are 

 called, in the order from before backward, the Antennae (feelers) 

 Mandibles, anterior and posterior Maxillae. 



Comparing these cephalic limbs with the analogous limbs of the Crustacea, we 

 see that in the Myriapoda and Iiisecta one pair of antennje is wanting. 



The cephalic limbs themselves are divided into 2 groups, the 

 feelers, and the oral limbs or mouth parts (mandibles and maxillae). 



The feelers (antennae) of the Myriapoda and Hexapoda are always 

 found in one single pair, and are pre-oral, springing from the forehead ; 

 they are long and slender, many jointed, very variously formed in 

 details, and very often different in the two sexes. They are organs of 

 touch, and at the same time carry the olfactory organs. They are 

 innervated from the brain. 



The oral limbs (mouth parts) vary extraordinarily in form, 

 according to the special functions to which they are adapted, these 

 functions being chewing, triturating, biting, sucking, and piercing, etc. 

 The tracing back of all these variously transformed mouth parts of 

 the Hexapoda to 3 pairs of oral limbs (mandibles, and anterior and 

 posterior maxillae) is one of the greatest achievements of comparative 

 anatomy. We can only take into consideration the principal forms of 

 these oral limbs. The mouth parts of the Orthoptera form the best 

 starting-point in our review, because in them the composition of the 

 lower lip (labium) of 2 lateral pieces (posterior maxillee) is most 

 evident. The whole apparatus (Fig. 303) is as follows. 



