INSECTA. 499 



each one forms one half of the antlia in Lepidoptera. In other sucking 

 insects the maxillae may be reduced to stilets, e. g. Rhynchota y may be lost 

 as in some Diptera, their palpi being either lost or preserved. The labium 

 is formed from the second pair of maxillae. The basal joints of the limbs 

 are united invariably, but the remaining parts may be more or less com- 

 pletely retained or lost. It possesses jointed labial palpi rarely absent. 

 In the Rhynchota it is modified into a four-jointed tube containing the 

 mandibles and maxillae. An upper lip or labrum, the nature of which 

 is not quite certain, forms a chitinoid plate well-developed in typical biting 

 insects, and lies transversely in front of the oral cavity in continuity with 

 the head. It usually bears on its oral surface an internal process or 

 epipharynx. The oral surface of the base of the labium also bears an 

 internal process or hypopharynx, beneath which the ducts of the labial 

 salivary glands open. The labrum and hypopharynx are produced into 

 stilets in some Diptera, and the fleshy proboscis of certain members of 

 this group (e. g. Muscidae) is formed from the hypopharynx and exterior 

 surface of the labium. Varieties of detail in the structure of the mouth- 

 parts are very numerous. 



The mouth-parts are attached to the head by articulation as well as 

 by muscles. In the Collembola (Apterygogened] as in Scolopendrella (see 

 p. 519) the mouth-parts, mandibles as well as maxillae, are retracted 

 within the head. They project from the head in other Insecta either 

 forwards in a direction prolonging the axis through the so-called occipital 

 foramen, or downwards in a direction at right angles to it. The insect in 

 the first case is said to be ortho- or pro-gnath, in the second hypo-gnath, 

 and the distinction applies equally to larva and imago. 



The thoracic limbs consist typically of a coxa, trochanter, femur, tibia, 

 and tarsus. The trochanter may be subdivided into two parts or fused 

 with the femur. The tibia is often armed with spines or calcaria, and the 

 tarsus is composed of a series of joints, usually not more than five in 

 number. The terminal joint bears two bent hooks or claws, between which 

 are often lodged one, two, or three membranous pads or pulvilli. Hairs 

 often clothe the under surface of the tarsal joints, and in jumping and 

 climbing Insects certain of them are connected to glands and exude a 

 sticky fluid. The form, size, &c. of the limbs depend upon the mode 

 of locomotion of the insect, e. g. running, swimming, jumping, &c. In 

 addition to limbs the meso- and meta-thorax in the majority of Insecta 

 give origin dorsally to wings. The Thysanura and Collembola appear to 

 represent a primitive group of Insecta in which wings have never been 

 developed ; hence Insecta Apterygogenea. Other Insecta may be designated 

 Pterygogenea, and wingless forms such as the Mallophaga, Siphonaptera, 

 &c. must be regarded as having lost their wings. The wings themselves 

 are primarily thin membranous expansions composed of two membranes, 



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