10 AN INTRODUCTION TO ENTOMOLOGY 



thorax; as a rule the young of mites have only six legs, but a fourth 

 pair is added during growth ; and in the gall-mites there are only four 

 legs. 



The Arachnida are air-breathing; but it is believed that they 

 have been evojved from aquatic progenitors. Two forms of respira- 

 tory organs exist in this class : first, book-lungs; and second, tubular 

 trachese. Some members of it possess only one of these types; but 

 the greater number of spiders possess both. 



A striking characteristic of the Arachnida, which, however, is also 

 possessed by the Palseostracha, is the absence of true jaws. In other 

 arthropods one or more pairs of appendages are jaw-like in form and 

 are used exclusively as jaws; but in the Arachnida the prey is crushed 

 either by the modified antennae alone or by these organs and other 

 more or less leg-like appendages. The arachnids suck the blood of 

 their victims by means of a sucking stomach; they crush their prey, 

 but do not masticate it so as to swallow the solid parts. 



In the Arachnida there exist only simple eyes. 



The reproductive organs open near the base of the abdomen on the 

 ventral side. In this respect the Arachnida resemble Limulus, the 

 millipedes, and the Crustacea, and differ from the centipedes and 

 insects. 



Among the more familiar representatives of this class are the 

 scorpions (Fig. n, a), the harvestmen (Fig. n, b), the -spiders (Fig. 

 ii, c), and the mites (Fig. n, d). 



As the writer has devoted a separate volume (Comstock, '12) to 

 the Arachnida, it will not be discussed farther in this place. 



CLASS PYCNOGONIDA 



The Pycnogonids 



The members of this class are marine arachnid-like arthropods, in 

 which the cephalothorax bears typically seven pairs of jointed appen- 

 dages, but in a few forms there are eight pairs, and in some the anterior 

 two or three pairs are absent; and in which the abdomen is reduced to a 

 legless, unsegmented condition. They possess a circulatory system, but 

 no evident respiratory organs. The reproductive organs open through 

 the second segment of the legs; the number of legs bearing these opening 

 varies from one to five pairs. 



The Pycnogonida or pycnogonids are marine animals, which bear 

 a superficial resemblance to spiders (Fig. 12). Some of them are 

 found under stones, near the low water line, on sea shores; but they 



