204 HIGH SCHOOL ZOOLOGY. 



pair of iDvancliing air-tubes, sucli as are universally present in 

 the insects, open further back near the spinnerets. These are 

 three pairs of pi-ojections, through short tubes on the ends of 

 which the spinning glands open. The secretion furnished by 

 these glands hai'dens on exposure to the air, and the threads so 

 formed are guided by the hind legs into the characteristic webs, 

 which serve as dwellings, or as traps for the prey, or even for 

 flight. 



18. In the Mites, on the other hand, the abdomen and cephalothorax are 

 coalesced and unsegmented, while the mouth-appendages are frequently 

 much modified by the adoption of a parasitic mode of life. Some of the 

 !Mites are parasitic on insects, during a larval stage, in which they have 

 only six legs, afterwards seeking their food on plants {Tromhldiam). 

 Some are aquatic forms, which may live free [Hydrachna), or parasitic- 

 ally on fresh-water mussels (Atax). Others are temporary parasites, like 

 the ticks {Ixodes), but there are various forma which live a pennanentlj- 

 parasitic life in the plumage of birds {Dermaldchtts), or in the skin of 

 mammals {Sarcoptes, Demodex). Finally, tlie cheese-mites and their 

 allies ( TijrogJijphus) have their mouth-parts adapted to the easy mode in 

 which they secure their food. Plants are not exempt from the attacks 

 of mites, for the species of one group (Plujtoptus), in which the two 

 hinder pairs of legs are rudimentary, make minute galls in the leaves 

 of various plants. 



The effect of the adoption of a parasitic mode of life is best seen in the 

 genus Fentastomiim, a form destitute of appendages, except for two pairs 

 of hooks near the mouth, wliich lives in the nasal cavities of Carnivora. 



With the exception of the larger mites ^nd ticks, all of the above 

 forms have no special respiratory organs, and this is the case also with 

 the bear-animalcules ( Tardujrada) ,a, group of microscoiiic creatures living 

 in moss, and feeding on minute larvae or Rotifers. Like the latter, they 

 may be desiccated and revived by moisture. They are associated with 

 the Araclmida on account of the number of appendages, but the fourth 

 pair of legs occupies the hinder end of the body. 



1 9. Apart from such minute air-breathing Arthropods as are 

 referred to in the above paragraphs, all have respiratory organs, 

 consisting (with the exception of the lungs of the scorpions and 

 spiders) of branched air-tubes, communicating with the outside 

 by " stigmata," and introducing air into all the tissues of the 



