17 

 INSECTS 



The number of different species of insect is enormous, probably 

 not less than 500,000 and equal to that of all other animals put 

 together. All insects resemble the cockroach in the main features 

 of their anatomy, but many of them depart widely from it in 

 detail, the differences affecting principally the mouth-parts, the 

 wings and the hfe-history. 



MOUTH-PARTS 



The full set of mouth-parts consists of three pairs of appendages 

 and three median structures. Segment four bears mandibles, 

 segment five maxillae, and segment six the labium. The median 

 structures above the mouth are the labrum, which is the most 

 anterior dorsal chitinous plate of the head and is capable of 

 some movement, and the epipharynx, which is the membranous 

 roof of the buccal cavity, often produced outwards and associated 

 with the labrum. Below the mouth is the hypopharynx or lingua, 

 which is a median process from the floor of the buccal cavity, 

 and bears the opening of the common salivary duct. The cock- 

 roach mouth-parts, which have already been described, have all 

 these parts, and their only important development is the fusion 

 of the basal portions of the labium to form the mentum, which 

 is common to all existing insects which possess the appendage 

 at all. The submentum is probably derived from the wall of the 

 head. Mouth-parts like these, with strong crushing mandibles, 

 are called mandibulate, and are characteristic of the more 

 primitive orders of insects ; they are also found in some members 

 of more advanced groups, especially in the larval stages. Besides 

 the cockroach and its relatives, other important insects which 

 possess them are the dragonfiies, beetles, and sawfiies. 



From this full and primitive set of mouth-parts adaptive radia- 

 tion (p. 474) has gone on in several directions to produce structures 

 suitable for different types of food. Often the same end is reached 

 by more than one means ; there are, for example, several different 

 modifications for sucking hquid food. On the whole, where the 



mandibles are well developed the maxillae are primitive, and parts 



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