APPENDAGES OF THE ARTHROPODA. 



245 



§ 189. 



Three pairs of tlie primitively similar ventral appendages are in 

 the Insecta converted into moutli-organs ; the same number are 

 formed into feet. The former, arranged round the mouth, may at 

 first have well served to seize and 

 hold food, just as we see the maxillas 

 of the Crab do at the present time. 

 In such a stage as this the food is 

 seized as well as comminuted. The 

 first pair form the mandibles, and 

 become parts of the mouth, in the 

 form of a single joint. The second 

 and third pairs are many-jointed. 

 But the basal joint only, or a few 

 of those succeeding it, which are 

 the nearest to the mouth, serve to 

 comminute the food ; these parts are 

 correspondingly metamorphosed. 

 They form the maxillae, and the 

 remaining portion of the appen- 

 dage looks like a jointed addition 

 to it, and functions as a tactile 

 organ (palp) ; in this way two 

 organs, which work in different 

 ways, are differentiated from one 

 appendage. 



The most indifferent form of 

 gnathite is found in the Aptera ; 

 in the Collembola they are sunk 

 into the buccal cavity, and in the Thysanura they are but feebly 

 developed. In the former when the parts of the mouth are in active 

 use they can be protracted and drawn in again ; and thus the mouth 

 is adapted both to biting and sucking, though, of course, to a 

 very small extent. This indifferent condition of the organisation is 

 developed along two distinct lines in the Pterygota. 



When the mandibles are well developed they have the form of 

 cutting organs, which work on one another; the two pairs of 

 maxillge also become cutting organs, and carry palps at the same 

 time. This condition is permanent in the Pseudoueuroptera, Neu- 

 roptera, and Orthoptera, although indeed points of similarity to the 

 more indifferent stage may be seen in the first of these ; and also 

 the second pair of maxillfe begins to show signs of fusion. When 

 these gnathites are fused in the middle line the so-called labium 

 is formed; its palps then become articulated to it as labial palps, 

 and indicate the more primitive condition of the organ. The 

 mouth-organs of the Coleoptera ai'e thus metamorphosed. 



These parts undergo more remarkable modifications when they 



126. Developmental stages of 

 Hyclrophilus picews. J. An earlier ; 

 B A later stage. Is Upper lip (labrum) . 

 at Antennco and first pair of gnathites 

 (Mandibles), mx Second pair (Max- 

 illa). U Third pair (Labium), p' p" 

 p"' Feet (after Kowalevsky) . 



