THE HEAD CAPSULE. 115 



or median eyes— ocelli— are situated. When it is closed by 

 plates of chitin these are usually, a triangular sclcrite some- 

 times obviously divided into three, which bears the ocelli, the 

 ' epifrontal sclerite ' {Mihi), and two flat plates below or in front 

 of the epifrontal, which I term ' mesofrontals.' The median 

 eyes, or ocelli, correspond very closely with the so-called pineal 

 eyes of the lower vertebrates, and just as the pineal eyes are 

 evanescent, so the median eyes of insects are frequently absent. 

 They are very rarely present in the Coleoptera ; but I have 

 one specimen of Cicindela maritima with a pair of ocelli, the 

 only specimen I have seen in which they are present. 



In the Muscidffi the frontal sac consists of a great part of 

 the posterior cephalocele withdrawn into the interior of the 

 head, between the meso-frontals and the antennal ridge, a 

 ridge developed by a process from each prefrontal lobe be- 

 tween the two cephaloceles. 



The anterior cephalocele persists in the Dragon-flies and in 

 the Homopterous Hemiptera as a very large sub-hemispherical 

 protuberance, but it is more generally closed by a pair of 

 plates, which subsequently meet in the middle line, forming 

 the facial plate. In front of the facial plate the profronta] 

 lobes meet in the middle line, and form the base of the upper 

 lip, or labrum ; this portion of the prefrontal lobes curves back 

 over the mouth. I term the whole of that part of the head 

 which is formed by the union of the prefrontal lobes in front of 

 the anterior cephalocele the 'prefacial region.' It is usually 

 distinctly divided into three parts— the meta-, meso-, and pre- 

 labrum. The former is termed the episteme; the mesolabrum 

 forms a rostrum in many insects; the prelabrum is commonly 

 known as the labrum. 



In the Diptera, at least, the pro-, meso-, and metalabrum are 

 generally each protected by a pair of lateral sclerites, which 

 are at first distinct, but become fused in the mature insect. 



It will be seen, therefore, that not only are there three 

 primary divisions of the cephalic nerve centres, excluding the 

 cephalic neuromeres, but three corresponding enlargements of 

 the primitive brain case or head capsule. Plate V. shows the 



8 



