126 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



[25] says there is no frontal sac in Volucella or the Syrphidae ; 

 in these insects a slight hemispherical swelling occupies its 

 place, and the lunula, instead of being a fissure, is a mere fold 

 in its lower margin. The head capsule in the flies is very large 

 in comparison to the cephalic nerve centres ; the greater part 

 of its cavity is occupied by large air-vessels, the much-folded 

 frontal sac, and the proboscis, when the latter is retracted. 



There is, apparently, a close relation between the size of the 

 frontal sac in the mature insect, and the power of retracting 

 the proboscis ; those forms in which the proboscis cannot be 

 retracted have apparently no persistent frontal sac. It appears 

 probable that the sac is compressed when the proboscis is 

 retracted, and distended when the proboscis is exserted. 



When the imago first escapes from the pupa the only 

 sclerites which are developed are the occipital ring and the 

 sutural ridges connected with it, the epioccipital plate, the 

 frontalia, the antennal ridge, the epifrontals, the mesofacials, 

 and sometimes the mesofrontals, as minute sclerosed points. 



The synonymy of the parts of the head capsule is as follows : 



Epicranium, Straus Durckheim. Upper head (Oberkopf). Burmeister. 



Paracephala, Mihi. Side of the head, Newport. Lateral plates of the 

 head, Mihi, 1870. Part of the epicranium, Straus Durckheim. They 

 include the occipitals of Menzbier. 



Frons, Menzbier. Vertex and frontalia of Robineau-Desvd. 



Face, Clypeii?, Hurmeiitcr. Antennal fossic, Robineau-Uesvd. Hypo- 

 stonia, Meigen and Bouchd. Clypeus posterior, Newport. 



Epicephalon, Mihi. Part of epicranium, Straus Durckheim and Menz- 

 bier. Cerebrale, Robineau-Desvd. 



Epistomum, Robineau-Desvd. Clypeus anterior, Newport. 



Metacephalon, Mihi. lialsband, Menzbier. 



Pars basilaris, Mihi. Submentum, or gula. 



Gula, Kirby and Spence. 



The terms ' pars basilaris' and 'gula' are usually considered synonymous. 



