440 TEXT-BOOK OF ENTOMOLOGY 



near the external plate. In the Hemiptera, also, the abdominal stig- 

 mata, though entirely free and visible, are situated ventrally. 



Primarily, in the embryo a pair of stigmata appear on each seg- 

 ment of the thorax and abdomen, except the 10th and llth, and 

 even possibly in the head, for a pair of stigmata are said to occur in 

 the head of Podurids (Smynthurus) (Lubbock), though this state- 

 ment needs confirmation. Scolopendrella, however, is known to pos- 

 sess a pair of cephalic spiracles. 



From the foregoing statement it will be seen that while in exist- 

 ing winged insects no more than 10 (in Japyx 11) pairs of stig- 

 mata are to be found in any one species, yet that 12 segments of the 

 body, in different groups taken collectively, bear them. The primi- 

 tive number of pairs of spiracles, therefore, in winged insects, was 12, 

 Ls. a pair in each thoracic segment, and a pair in each of the first 

 nine abdominal segments. Insects were originally all holopneustic, 

 and gradually as the type became differentiated into the different 

 orders they became peripneustic or amphipneustic, and, in certain 

 aquatic forms, apneustic. (See pp. 459, 461.) 



In the still more primitive, probably wingless, ancestors of insects 

 there was a larger number of stigmata. Hatschek, in 1877, discov- 

 ered a pair of tracheal imaginations in each of the three posterior 

 head-segments of the embryo of a moth, with stigmatal openings 

 in the 1st and 2d maxillary segments. 



Thus early in embryonic life every segment of the body, except 

 those bearing the eyes and the last abdominal, bore a pair of stig- 

 mata, so that the primitive insect had at least 15, and perhaps more, 

 pairs of stigmata. 



The position of the stigmata is subject to much variation, the 

 result of adaptation to this or that mode of life. Examples are 

 those insects which live in dusty situations or usually more or less 

 concealed in the earth, as in most beetles, and in the Hymenoptera. 

 In such beetles, the stigmata are situated in the thin membrane 

 between the segments ; in the Hymenoptera, on the upper edge of 

 the segments. In the Siphonaptera, Pediculina, bedbug, and similar 

 forms, which breathe an air freer from dust, the spiracles lie free on 

 the outside of the body. 



"When the stigmata are free and without any protection on the abdomen, 

 there are other ways by which the entrance of foreign bodies into the tracheae is 

 prevented. In such cases the body is covered with dense hairs, as in most Dip- 

 tera and Neuroptera, as well as many Lepidoptera ; or there is situated in front 

 of the stigma either a small tissiye which is covered over by a number of hairs 

 arising from the edge, as in many Orthoptera ; or, as in most insects, a luxurious 



