66 MEDICAL ENTOMOLOGY 



The wall of the head capsule is not thrust forward sufficiently in this 

 species to justify the term rostrum, but the same sort of movement 

 occurs. The invagination of the membrane around the haustellum 

 is undone as the piercing stylet is thrust out between the diverging 

 palps. (Plate XI, fig. 3.) 



The relations of the muscles are so like those in Musca that the figure 

 will explain itself. 



INTERNAL STRUCTURES OF THE HEAD 



The only internal structures connected with the exo-skeleton of the 

 head in the Diptera are the intracranial tunnels and the ptilinum. 

 The former occur only in the Orthorraphic flies, while the latter is 

 characteristic of the Cyclorrappha. 



The intracranial tunnels (Plate IV, fig* 4) are probably homologous 

 with the tentorium of other insects. They occur in all the blood- 

 sucking Orthorrapha, and in many of the others. In 

 Intracranial Tunnels . . 



most cases they are hollow tunnels, passing from the 



anterior to the posterior wall of the head capsule, and open at each 

 end. They are bilateral, those of the two sides converging towards one 

 another from behind forwards, on each side of the first part of the suck- 

 ing apparatus and a little above it. The posterior opening is situated 

 between the base of the proboscis and the occipital foramen, and 

 the anterior at the side of the clypeus, below the antennae. The 

 tunnel is generally narrowest in its middle position, and in some 

 forms, as for instance Simulium, the lumen is closed at the nar- 

 rowest point. 



The function of these tunnels is to support the walls of the head 

 cavity, and especially to enable them to resist the action of the powerful 

 dilator muscles connected with the sucking apparatus. In Tabanus they 

 are so placed as to transfer the strain of the muscles of the buccal cavity 

 from the anterior to the posterior wall of the head, and, in order to 

 distribute the strain over a wider area, the anterior end expands in a 

 funnel-shaped manner. The posterior end is attached to a strong bar of 

 chitin at the side of the membraneous area of the inferior wall, at the 

 base of the proboscis. The fact that they are hollow has no more 

 significance than that, weight for weight, a hollow cylinder is stronger 

 than a solid rod. 



The submental region, which in the Orthorraphic flies is mostly 

 membraneous, is strengthened by the development of strong lateral bars 



