ARTICULATA. 231 



to keep out foreign substances, communicate with 

 branching tubes, whose membraneous wall is strength- 

 ened and kept open by a coiled spinal filament. (Fig. 

 132.) What are called the " nerves" of an Insect's wing 

 are double tubes, the inner one being a tracheal branch 

 supplying air, and the outer one, sheathing it, is a blood- 

 vessel. 



The mouth of an Insect is a very complicate appara- 

 tus. Some are Masticatory, or fit- 

 ted for biting, as in Beetles. (Fig. 

 133.) Others are Suctorial, or for 

 sucking, as in Butterflies. These 

 last form a long double tube, or 

 spiral trunk, (proboscis,) serving to 

 pump up the juices of flowers. 

 The masticating mouths consist of 

 two pairs of horny jaws, (mandibles FIG. i 33 . Masticatory mouth 



, // \ 1 1 i 1 of Insect. <z. Labrum, or upper 



and maxilla,) which work horizon- lip . 3. Labium, or lower H P , 



tally between an upper (labrunf) lip 



and an under (labium) lip. The bles< 



maxillae and under lip carry sensitive jointed feelers, 



(palpi.) The front edge of the labium is generally 



known as the tongue, (ligula.) 



In the Bee tribe, instead of maxillae, we find a sheath 

 inclosing a long, slender, hairy tongue. Entomologists 

 have retained the same names to the different parts, 

 under the influence of the theory of transmutation. 

 (Fig. 134.) 



The proboscis of the Fly is an enlarged lower lip, 

 (Fig. 135;) that of the Bugs is formed by four bristles, 

 fitted both for piercing and sucking. 



