THE MUSCID HEAD 15 



structure of the wall is modified. The most striking and indeed the 

 characteristic feature of the head in this group is the presence on the 

 front of a long arched suture, commencing transversely above the 

 antennae and extending downwards on each side of them, almost to the 

 distal border. This is the ptilinal suture, and marks the place at which 

 the ptilinal sac was pushed out at the time of the emergence of the 

 imago from the puparium. The shape of the head is further modified 

 by the increase in size of the basal joints of the antennae, which are pen- 

 dulous, and rest in hollows on the anterior surface. The base of the head 

 is also altered by the fact that a part of the wall has become merged with 

 the proboscis, as will be explained shortly, and has become retractile. 

 When the head is viewed from behind the whole of the posterior wall is 

 seen to be chitinous, while at the lower border there is a rounded foramen 

 between the anterior and posterior surfaces, through which the proboscis 

 is protruded. The gular region has, therefore, become merged into the 

 posterior wall of the first part of the proboscis. The aperture through 

 which the latter is pushed out is sometimes referred to as the buccal 

 orifice, a most unfortunate term, for it is some distance posterior to the 

 structure universally known as the pharynx, and has nothing whatever to 

 do with the mouth parts. It is better to use the term epistomal orifice, 

 as the thin plate of chitin in front of it is known as the epistoma. 



The terms used in describing the Muscid head (Plate III, fig. 1) are 

 somewhat perplexing, mainly on account of their multiplicity. The area 

 enclosed by the lateral arms of the ptilinal suture is known as the 

 ptilinal area, the facial depression, or the front. It is almost entirely 

 made up by the clypeus, for which alternative names are the face, facial 

 plate, mesofacial plate. The antennae may lie in deep or shallow 

 grooves, termed the fossae of the facial plate, or the antennal grooves. 

 The grooves may be separated by a ridge, termed the facial carina, and 

 this may be produced forwards in its lower portion. The lateral margins 

 of the ptilinal area may be thickened into ridges, termed the facialia, 

 or the facial ridges, or vibrissal ridges. They terminate in pro- 

 minent angles known as the vibrissal angles. Below the facial 

 plate or clypeus there is a small mesial plate, forming the boundary of 

 the epistomal orifice, and termed the epistoma. The orifice itself, 

 through which the upper part of the proboscis passes out in extension, 

 is also known as the oral orifice or buccal orifice, and an oral cavity 

 is mentioned, supposed to be bounded in front by the epistoma. 

 Below the epistoma on the anterior surface there is a transverse plate 

 regarded by many authors as the clypeus, the upper and larger plate 



