INSECTA. 405 



thorax consists of three segments, each of which carries a pair 

 of legs. Mostly there are two pairs of wings borne by the two 

 hinder segments of the thorax. The abdomen never carries 

 locomotive limbs, but the last abdominal segments may carry 

 reproductive or sensory appendages. A single pair of jointed 

 antennae is present, and the eyes are generally compound. 

 Respiration is effected by air-tubes (trachece). 



The number of Insects known from Tertiary deposits is 

 already extremely large, while they are comparatively scarce 

 in the Secondary period, and are among the very rarest 

 of fossils from the Palaeozoic formations. Moreover, the 

 remains of this class of animals almost always occur in a 

 fragmentary condition, and under any circumstances they 

 cannot be satisfactorily deciphered except by practised 

 workers in the department of Entomology. For these 

 reasons, among others, it will be sufficient to give here a 

 brief outline of the geological history of the class. 



As regards the general distribution of the Insecta in time, 

 the oldest -known forms are from the Devonian rocks of 

 North America. Here occur 

 the remains of several insects 

 which belong to the order 

 of the Neuropterous Insects 

 (or to the Pseudoneuroptera). 

 Amongst the most remark- 

 able of these is the Plate- 

 phemera antiqua of Mr Scud- 

 der (fig. 256). This species 

 must have attained a large 

 size five inches in expanse of wing and it is regarded by 

 Mr Scudder as being referable to the Ephemeridce (the May- 

 flies). This eminent authority, however, regards it as a 

 " synthetic type " that is to say, as a form combining pecu- 

 liarities of structure which are now only found in different 

 groups. Other genera belonging to the Neuroptera have been 

 described from the Devonian rocks of North America, under 

 the names Homothetus, Lithentomum, and Xenoneura, the last 

 having attached to its wing the remains of a " stridulating 

 organ," similar to that possessed by recent grasshoppers. 



