SEGMENTATION OF THE HEAD 7 



these writers conclude that the labrum represents the first true 

 cephaHc segment. 



Whatever may be the true interpretation of the foregoing 

 observations, it needs to be recollected that in the Collembola and 

 in Lepisma, Campodea and Japyx the labrum develops as an 

 unpaired rudiment from its first stage of growth. The same 

 condition is found in Scolopendra and also in Blatella, Mantis, 



Fig. 3. Portion of a longitudinal vertical section of a fifty-nine 

 hours' old embryo of Locusta, showing the labral ccelom. 

 X 375. a, amnion ; I, labrum ; Ic, labral coelom ; m, mesoderm ; 

 s, stomodaeum. (After Roonwal.) 



Gryllus and Forficula. On the other hand, the labrum develops 

 from paired lobes in Carausius, Locusta, Stenobothus, Rhodnius 

 and in Lepidoptera and Coleoptera. Among Hymenoptera its 

 rudiments may be either paired (Chalicodoma) or unpaired (^pf^). 

 The labrum, therefore, may differ as to its mode or origin within 

 the limits of a single order of insects. We are thus faced with the 

 difficulty of deciding which of these two conditions is to be 

 regarded as the original or primary state. Since the labrum 

 develops from unpaired rudiments in the Chilopoda and 



