80 



[CH. 



struggles of the latter become less violent, the flexor muscles come 

 into play, and draw the mask back to its position of rest ; so that 

 the mandibles can be brought into action. A more effective or 

 more cruel implement it would be almost impossible to conceive 

 for its purpose. 



Homologies of the Labial Mask. Considerable discussion has 

 centred round this point. Our description has followed the views 

 of Butler [19], who supports them with sound ontogenetic evidence. 

 Her views agree with those originally advanced by Rambur and 

 Hagen. The opposing view is that of Gerstaecker [55]. He con- 

 siders that the lateral lobe represents the fused palpus and galea, 

 while the ligula (median lobe) is formed of the fused laciniae only. 

 His argument is much weakened by the well-known fact that 

 parallel developments of the first and second maxillae occur very 

 frequently. In the Odonata, the inner lobe of the first maxillae 

 is formed by the fused galea and lacinia. We should expect a 

 parallel condition in the labium. 



Accepting Butler's views, we may hypothetically construct 

 (fig. 31) a typical archaic form of 

 Odonate labium, from which all present 

 types, both of larval and imaginal 

 forms, can be easily and simply derived. 

 This archaic labium has, of course, the 

 exact structure of the generalized type 

 of labium found in mandibulate insects 

 (e.g. the Cockroach), and corresponds 

 with a fused pair of typical biramous 

 jointed appendages of the Crustacea. 

 It has a pair of fused two-jointed basal 

 protopodites, which carry distally both 

 endopodites (ligula) and exopodites 

 (palpi), the latter being two-jointed 

 also. The following table gives the 

 parts of such a labium, and their re- 

 spective fates in the larval and imaginal 

 labia of recent Odonata : 



ex 



P 1 



Fig. 31. Hypothetical archaic 

 labium of Odonata, to shew 

 homologies of parts, a apex 

 of lateral lobe ; d distal border 

 of same ; ex exopodite ; g ga- 

 lea ; i inner border of lateral 

 lobe; Ic lacinia; lig ligula 

 (endopodites) ; ra mentum ; 

 ml median lobe; o outer 

 border of lateral lobe; p lt 

 p z joints of palpus ; pr pro- 

 topodite; sm sub-mentum. 

 Original 



