44 CLAUDE FULLER. 



nascent stage of the respiratory system. Of the four, fig. 20b 

 is an external aspect of the arrangement upon the left side 

 of the body, and forms the basis of the diagrams used to 

 illustrate the homologies. Fig. 20a (PI. IV), is more or less 

 diagrammatic (see Explanation of Plates). 



It is not difficult to resolve the system of the trunk into 

 a regular scheme by alterations of a minor nature ; the 

 abdominal spiracular loops are straightened to accord with 

 their appearance in the winged imago, wherein they seem to 

 stretch taut from spiracle to spiracle ; the series of palisade 

 and ventral commissures are also altered to accord with one 

 another, the two palisades of the thorax beiug somewhat 

 lengthened to agree more in height with the rest. Thus, the 

 system of the head, neck, and prothorax can be first resolved 

 into the scheme given in fig. 46 (PI. V), iii which it is shown 

 connected with the trunk system. This rearrangement of the 

 trachea? enables us to see (1) that the basal part of the 

 cephalic dorsal (or antenna!) trachea is a palisade commissure 

 that has been thrown down and inflated ; (2) that the anterior 

 vertical commissure (connecting the dorsal trachea with the 

 stem of the great U-shaped trachea) may be regarded as a 

 palisade commissure, and that the stem of the U is the con- 

 tinuation of the spiracular trunk. The first inference to be 

 drawn is that the arch of the U is a modified ventral com- 

 missure. If this is the case^ then there are two pairs of 

 palisade commissures, and, counting the cervical as one, two 

 ventral commissures which ai'e not associated with spiracles. 

 On this it may be assumed that two spiracles have been lost 

 to the system. It is obvious that the ineso- and metathoracic 

 leg-trachea^, fig. 9 (PI. Ill), owe their Y-shaped form to the 

 anastomosis of two trachese. In view of the separation of 

 the tracheae in the tibia of the front leg, fig. 16 (PI. Ill), it is 

 reasonable to assume that the base of this was also composed 

 of two tracheae. The commissure of the neck connects the 

 tracheae of the first pair of legs some distance below their 

 attachment to the spiracular trunk ; but it is not unreasonable 

 to suppose that it was originally attached to the spiracular 



