ii4 THE I NTEGU MENTAL SKELETON OF THE IMAGO. 



The pre-oral part is either developed from a cap of blasto- 

 derm, the head-cap, or from a single pair of imaginal discs, 

 which represent the invaginated head-cap and procephalic 

 lobes. The structure of the head is also the same, whether it 

 is developed from the blastoderm directly, or from the invagi- 

 nated imaginal discs. 



The procephalic lobes (PI. V., pa c} are thickened lateral 

 portions of the head-cap, and are connected with the segmented 

 post-oral primitive band by a pair of crura which surround 

 the stomodseal pit. The procephalic lobes bear the great 

 eyes (e) and the antennas (a), which are undoubtedly olfactory 

 organs. 



The head capsule will be seen therefore, to consist of a 

 segmented portion behind, which I term the ' metacephalon,' 

 and laterally of two plates in front of the metacephalon, bear- 

 ing the great eyes and the antennae; these I term 'paracephala' 

 (PL V., pa c). The paracephala are united in front, and 

 form the epistomum and the labrum (pi}. The median region 

 behind the epistomum, or metalabrum which I regard as a 

 preferable term, exhibits three distinct parts ; two of these 

 are frequently bladder-like swellings, the anterior and posterior 

 cephalocele (PL V., a cc and p cc}. The facial region is developed 

 from the former and the forehead from the latter, the simple 

 eyes are situated in the forehead, or on the posterior cephalo- 

 cele. Behind the forehead there are two plates, which extend 

 forwards from the metacephalon ; these form the epicephalon. 



That portion of the procephalic lobe which lies in front of 

 the crus unites with its fellow, and curves downwards and 

 backwards over the mouth, forming the prefacial region 

 (mes I and pi}. So that there is a close resemblance between 

 the crura of the profrontal lobes and the trabeculae cranii of 

 the vertebrate. 



The anterior and posterior cephalocele correspond with the 

 thin portion of the blastoderm which intervenes between the 

 procephalic lobes. The posterior cephalocele is the fore-head 

 (Vorderkopf} of the German embryologists. In the Dragon- 

 flies it persists as a bladder-like swelling, on which the simple 



