250 



INSECTS. PHYLUM ARTHROPODA 



parallels that of the true ants, to which they are not nearly 

 related. Most of the individuals are sterile and wingless workers 

 and soldiers of both sexes. Each colony is founded by a royal pair, 

 the king and queen, which at first have wings, but they lose these 

 when they settle down in matrimony. Many termites feed solely 

 on wood." but they can only do so with the help of symbiotic 

 flagellate Protozoa which inhabit the gut and break down the 

 lignin. rixperimentally termites can Hve on the purest cellulose 

 known, and the source from which their symbionts obtain nitrogen 

 remains a mystery. The mouth-parts are biting. 



Order 8. Plecoptera. The stoneflies have biting mouth-parts 

 and aquatic larvae (naiads) with tracheal gills— expansions of 



^Uppc- lip labrum 

 • Antennae 



'Compound Eyes 



Winq Cases 



Mask 

 Fig. 1 8 1. — Dragonfly larva. — From Sandars. 



the body- wall richly supplied with tracheae. Perla carlukiana 

 ( = marginata) is common by English streams. 



Orders 9, Embioptera, and 10, Psocoptera, are small orders of 

 small insects, with many apterous forms. Trogiiim (=Atropas) 

 pulsatorium, the book-louse (Psocoptera) is notorious for living 

 on the paste of book-bindings, but it is not confined to this diet. 



Order 11. Odonata. The dragonflies are large insects with 

 biting mouth-parts, predacious in both larval and adult stages ; 

 the eyes are big and the antennae reduced. The larvae are naiads 

 with tracheal gills either at the posterior end of the body or in the 

 rectum, and a characteristically modified labium known as the 

 mask ; this is normally carried folded under the head, but can be 

 suddenly shot out to seize an animal such as a tadpole. 



Order 12. Hemiptera or Rhynchota. These are generally known 

 as bugs, although they include many species, such as the water 

 scorpion (Nepa) and water boatman [Notonectd], to which the 

 term is seldom applied. All have piercing and sucking mouth-parts 

 of the same general type as those of the aphis, which has been 



