I 12 



INSECTS 



leading into a chamber, the atrium, which is limited inwardly 

 by the occluding apparatus ; and beyond this there is a second 

 chamber, the vestibule, separated from the tracheae proper by a 

 valvular arrangement. He considers that the vestibule acts as a 

 pump to force the air into the tracheae. 



Systematic Orientation. 



Terms relating to position are unfortunately used by writers 

 on entomology in various, even in opposite senses. Great 

 confusion exists as to the application of such words as base, apex, 

 transverse, longitudinal. We can best explain the way in 

 which the relative positions and directions of parts should be 

 described by reference to Figure 62. The spot 3 represents an 

 imaginary centre, situated between the thorax and abdomen, to 

 which all the parts of the body are supposed to be related. The 

 Insect should always be described as if it were in the position 

 shown in the Figure, and the terms used should not vary as the 

 position is changed. The creature is placed with ventral surface 

 beneath, and with the appendages extended, like the Insect itself, 

 in a horizontal plane. In the Figure the legs are, for clearness, 



made to radiate, but in the 

 proper position the anterior pair 

 should be approximate in front, 

 and the middle and hind pairs 

 directed backwards under the 

 body. The legs are not to be 

 treated as if they were hanging 

 from the body, though that is 

 the position they frequently 

 actually assume. The right and 

 left sides, and the upper and 

 lower faces (these latter are 

 frequently also spoken of as 

 sides), are still to retain the 

 same iromenclature even when 

 the position of the specimen is 

 reversed. The base of an organ 

 is that margin that is nearest 

 that which is most distant. 



Fig. 62. — Diagrammatic Insect to explain 

 terms of position. A, ape.x ; B, base : 

 1, tibia ; 2, last abdominal segment ; 3, 

 ideal centre. 



to the ideal centre, the apex 



