THE INTERNAL ANATOMY OF INSECTS 115 



second and third thoracic segments and the first to the eighth abdom- 

 inal segments, respectively. There are none in the corresponding 

 position in front of the first thoracic segment. See account of 

 cephaHc silk-glands p. 103. 



The two pairs of thoracic spiracles are commonly distinguished as 

 the mesothoracic and the metathoracic spiracles ; that is each pair of 

 spiracles is attributed to the segment in front of which it is normally 

 situated. Following this terminology there are no prothoracic 

 spiracles; although sometimes the first pair of spiracles is situated in 

 the hind margin of the prothorax, having migrated forward from its 

 normal position. It would be better to designate the thoracic 

 spiracles as the first and second pairs of thoracic spiracles, respec- 

 tively; in this way the same term would be applied to a pair of 

 spiracles whatever its position. There are many references in 

 entomological works to "prothoracic spiracles," but these refer to the 

 pair of spiracles that are more commonly designated the mesothoracic 

 spiracles. 



In many cases the abdominal spiracles have migrated back upon 

 the segment in front of which they are normally situated, being fre- 

 quently situated upon the middle of the segment. 



The statements made above refer to the normal number and dis- 

 tribution of spiracles; but a very wide range of variations from this 

 type exists. Perhaps the most abnormal condition is that found in 

 the genus Smi nthurus of the Collembola, where there is a single pair 

 of spiracles which is borne by the neck. In the Podurid^, also of the 

 Collembola, the respiratory system has been lost, there being neither 

 tracheas nor spiracles. 



Terms indicating the distribution of the spiracles. — The following 

 terms are used for indicating the distribution of the spiracles; they 

 have been used most frequently in descriptions of lan^as of Diptera. 

 These terms were formed by combining with pneustic (from pneo, to 

 breathe) the following prefixes: peri-, around, about; pro-, before; 

 meta- after; and aniphi, both. 



Peripneustic. — Having spiracles in a row on each side of the body, 

 the normal type. 



Propneustic. — ^With only the first pair of spiracles. 



Metapneustic. — With only the last pair of spiracles. 



Amphipneustic. — With a pair of spiracles at each end of the body. 



