THE HEAD 13 



Audoin supposed that the exo-skeleton of a primitive segment was 

 composed of eight chitinous plates, arranged in pairs, two dorsal, two 

 ventral, and two lateral plates on each side. Although this is not 

 generally accepted as a type to which the very numerous and diverse 

 forms may all be referred, his nomenclature has been preserved, with 

 some modifications. He termed the dorsal plates the terga, the ventral 

 the sterna, the pair of lateral plates contiguous with the terga the 

 epimera, and the pair contiguous with the sterna the episterna. The 

 two lateral plates when fused together are known as the pleura. Such 

 a primitive arrangement is not to be looked for in the Diptera, and it 

 is best to regard the terms as descriptive only, without laying too 

 much stress on their significance, especially since hardly any two authori- 

 ties are agreed on the point. In the head the various segments have 

 become fused together to form a chitinous box, often termed the head 

 capsule, and few if any really reliable indications of the original segmenta- 

 tion are to be found in the adult insect. In the thorax a nearer approach 

 to the type is found, but on account of the development of extra plates, 

 and the displacement of others, it is very difficult to refer them to 

 the type, especially in the case of the lateral plates. In the abdomen the 

 arrangement is simple, as the terga and the sterna are preserved, and the 

 interval between them occupied by a soft membrane, which, as it 

 represents the pleural plates and bears the openings of the breathing 

 tubes, is termed the pleural membrane. 



Each chitinous plate is separated from its neighbour by a narrow 

 interval which is filled in either by soft membrane or by thinner chitin, 

 and in this way the whole body of the insect is provided with a series of 

 joints, which enable the various segments to move on one another. The 

 head is united to the thorax by a flexible neck ; the segments which make 

 up the thorax are firmly welded together for the most part, but there is 

 a membraneous interval in the lateral wall which permits of a certain 

 amount of expansion ; in the abdomen each dorsal and each ventral plate 

 is connected with those in front of and behind it by means of a narrow 

 strip of membrane. The dorsal plates are connected with the ventral 

 ones by the pleural membrane. 



THE HEAD (Plates I and III) 



The head capsule is a rounded and compact box, composed of the chiti- 

 nous plates which belong to the exo-skeleton of the first five* segments 



* Of the six segments which should go to make up the head, the third is absent in the 

 majority of insects. 



