The Internal Anatomy of Spiders 



The muscles extending backward from the third abdominal 

 endosternite are those that move the spinnerets. 



THE RESPIRATORY ORGANS 



In the order Araneida, two types of respiratory organs are 

 found; first, the book-lungs; and second, the tubular tracheae. 

 Some members of the order possess only one of these types; but 

 the greater number of spiders possess both. 



The number of respiratory organs is reduced in spiders to 

 two pairs; and in most true spiders the second pair open by a 

 single spiracle on the middle line of the body. 



The more generalized spiders preserve two pairs of book-lungs. 

 This condition is retained by Liphistius, all of the tarantulas, 

 and by a single family of the true spiders, 

 the Hypochilidae. Excepting the Hypochi- 

 lidae, most true spiders have a single pair 

 of book-lungs; but in the Caponiidae, which 

 is not represented in the United States, 

 both pairs of respiratory organs have been 

 transformed into tubular tracheae. 



The accompanying figures (Fig. 152, 

 1-4) show in a diagrammatic way the dis- 

 tribution of the two types of respiratory 

 organs among the families of spiders; in 

 these figures the tracheal spiracles can be 

 distinguished from lung-slits by the absence 

 of external indications of book-lungs. The 

 distribution is as follows: 



1. With two pairs of book-lungs; 

 Liphistius, the tarantulas (Avicularioidea), 

 and the Hypochilidae. 



2. With one pair of book-lungs and 

 one pairof tracheal spiracles; the Filistatidae, 

 the Oonopidae, and the Dysderidae. 



3. With one pair of book-lungs and a single tracheal spiracle; 

 all true spiders (Argiopoidea) except the Hypochilidae, Filistatidae, 

 Oonopidae, Dysderidae, and Caponiidae. 



4. With two pairs of tracheal spiracles; the Caponiidae. 

 The hook-lungs. — These are sacs filled with air each of which 



open by a slit-like spiracle on the ventral aspect of the abdomen 



Fig. 152. DIAGRAMS 



SHOWING THE 



DISTRIBUTION OF THE 



TWO TYPES OF 

 RESPIRATORY ORGANS 



1, Liphistius, the taran- 

 tulas, and Hypochilidae 



2, Filistatidu.-. Oonopidae, 

 and D : rid 



3, all true spiders except 

 the five families men- 

 tioned here 



4, Caponiida: 



145 



