GENERAL NOTES ON INSECTS. 303 



we can fancy that an increase in respiratory efficiency brought about by 

 the outgrowths in question would quicken the whole life, and would 

 tend to raise insects into the air, just as terrestrial insects can be made 

 to frisk and jump when placed in a vessel with relatively more oxygen 

 than there is in the atmosphere. Finally, we must note that the 

 aquatic lame of some insects, e.g. may-flies, have a series of respiratory 

 outgrowths from the sides of the abdomen, the so-called ',' tracheal 

 gills," which in origin and appearance are like young wings (Fig. 134). 



Insects excel in locomotion. " They walk, run, and jump 

 with the quadrupeds ; they fly with the birds ; they glide 

 with the serpents, and they swim with the fish." They beat 

 the elastic air with their wings, and though there cannot be 

 so much complexity or movement as in birds where the 

 individual feathers move, the insect wing is no rigid plate, 

 and its up-and-down motions are complex. They can soar 

 rapidly, but their lightness often makes horizontal steering 

 difficult. The wind often helps as well as hinders them ; 

 thus the insects which fly in and out of the windows of 

 express trains are probably in part sucked along. Marey 

 calculates the approximate number of wing strokes per 

 second at 330 for the fly, 240 for the humble-bee, 190 for 

 the hive-bee, no for the wasp, 28 for the dragon-fly, 9 for a 

 butterfly. For short distances a bee can outfly a pigeon. 



Skin. — As in other Arthropods, the epidermis (or hypo- 

 dermis) of Insects forms a firm cuticle of chitin, which in 

 the exigencies of growth has sometimes to be moulted. 

 This cuticle is often finely marked, so that the animal seems 

 iridescent ; and there are many different kinds of scales, 

 hairs, and spines. Chitin is not favourable to the develop- 

 ment of skin glands. Most insects have " salivary glands " 

 opening in or near the mouth. Bees have wax-making glands 

 opening on the abdomen ; aphides have glandular tubes ; 

 not a few have poison bags ; and many larvae besides silk- 

 worms have organs from which are exuded the threads of 

 which a cocoon is made. 



Muscular system. — In very active animals like Insects, 

 we of course find a highly developed set of rapidly contract- 

 ing striped muscles. These work the wings, the legs, and 

 the jaws. The resulting movements have this further 

 significance, that they help in the respiratory interchange of 

 gases, and in the circulation of the blood. 



Nervous system. — It is often remarked as marvellous 



