AA^A TOM Y AND MORPHOLOG Y OF INSECTS. 1 1 



as important in relation to classification.* The very early 

 development of the setse in the nymph is an indication in 

 favour of his view. 



The Diptera generally, and the Muscidae more especially, 

 exhibit two very distinct modifications, which Osten-Sacken calls 

 the Diptera chaetophora and the Diptera eremocheta. The 

 former are densely covered with bristles, and use their legs to 

 run, climb, and snatch their prey ; their flight is headlong, and 

 they rarely or never poise over flowers. The olfactory sense 

 appears to predominate. On the other hand, the latter hover 

 and poise on the wing. They are smooth and have no bristles, 

 or very few. Many are brightly coloured. The visual sense 

 appears to predominate, and they only use their legs to alight 

 and when resting. They are helpless in the dark, and are 

 seldom abroad except on sunny days. Dr. A. Forelf says that 

 ' insects organized for an exclusively aerial life depend on their 

 eyes. They generally have little developed antennae, and are 

 absolutely helpless in the dark ; they hardly dare to walk. . . . 

 In other insects the eyes play a very subordinate part. These 

 may be called antennal insects. They can work by night or 

 underground as well as by day.' 



The blow-fly belongs to the chastophorous division, and is in 

 some degree an antennal insect in Forel's sense, but it has 

 evidently good visual powers. 



Apodemes arc rod-like involutions of the cuticle, to which 

 muscles are attached. They are frequently very strong, and act 

 as powerful levers. 



The Thorax in the imago is always provided with three pairs 

 of jointed limbs. The anterior pair are sometimes rudimentary. 

 It consists entirely or in great part of three metameres, known 

 respectively as the prothorax, the mesothorax, and the meta- 

 thorax. 



Tracheae. — Insects always possess a peculiar respiratory 



* Osten-Sacken, C. R., 'An Essay on Comparaiive Chatotaxy.' Trans. 

 Entom. Soc, London, 1884. Originally primed in iMilth. der Miinchener 

 Entom. Vereiiis, lid. v., 1881. 



t A. Forel, ' Beitra<j z. Kcnntniss der Sinnesempfindung der Insecten.' 

 Mittli. d. Miinchener Entom. Vereins, ii., 1878. 



