4i6 



THE BIOLOGY OF INSECTS 



action of the feeding mites, which may multiply to such an 

 extent that they choke the cavities. Many bees thus 

 infected lose their power of flight, and the ** Isle of Wight 

 disease," of which this inabihty is a conspicuous symptom, 

 is now generally ascribed to Acarapis rather than to the 

 Nosema mentioned above. It is easy to believe that the 



damage and obstruction 

 which these mites cause to 

 the thoracic air-tubes may 

 disturb the action of the 

 wing-muscles by reducing 

 their supply of oxygen ; 

 nevertheless some bees with 

 a heavy infestation are able 

 to fly very well, so the 

 subject clearly calls for 

 further investigation. 



Besides the mites just 

 described that are parasites, 

 other members of the same 

 order may be found attached 

 to the bodies and legs of 

 insects, flies, for example, 

 not that they may suck food, 

 but that their race may have 

 lusc (Sp/jam'um) grasping its hind the advantage of wide dis- 

 persal. The Tyroglyphidae 

 are a family of mites well-known to many persons, through 

 the abundance of one or two species in cheese and stores of 

 meal and hay. In their life-history there is a curious nymph- 

 stage known as the hypopus — a form with flattened firm- 

 coated body, beneath which the slender legs are inserted close 

 together, being provided with sucker feet by means of which 

 the little creatures maintain attachment to the flies or other 

 insect-carriers on which they take opportunity to fasten. 

 The sudden apparently inexplicable appearance of swarms 

 of Tyroglyphi in a store of meal is due to the transport of 

 hypopus-nymphs which, leaving hold of their fly carriers. 



Fig. 85. — ^Water - scorpion {Nepa 

 cinered) with small bivalve mol- 



