238 INVERTEBRATE ZOOLOGY 



the imago fairly closely resemble the general bodily structure 

 of the imago (Fig. 113) and are termed nymphs. The lack of 

 functional wings usually differentiates the nymph from the adult, 

 but external wing pads from which the wings later develop are 

 characteristic of many nymphs. 



The Thysanura and CoUembola are wingless and throughout 

 life retain essentially the forms they have at hatching. There 

 are some changes, but these are so inconspicuous that many are 

 inclined to refer to the insects of these two orders as the 

 Ametabola. 



Appendages of the Head. — The head of an insect bears a single 

 pair of antennae, the eyes, and the mouth parts (Fig. 114). 

 These last comprise an unpaired labrum or upper lip, a pair of 

 mandibles, the hypopharynx, the maxillae, and the labium. 

 The labium, or lower lip, is in reality a second pair of maxillae 

 of which at least the basal portions are fused along the median 

 line. Both the maxillae and the labium are composed of several 

 distinct sclerites and bear palpi, designated respectively as 

 maxillary and labial palpi. All of the appendages of the head 

 are articulated with immovable parts, forming the head capsule. 



Modifications of Mouth Parts.^ — The mouth parts are subject 

 to numerous modifications of form and function. As described 

 above, they are suited for holding and chewing food, but in many 

 groups only liquid food is talcen and in these groups some of 

 the mouth parts are modified to form a sucking tube. The 

 most significant of these suctorial modifications are found in the 

 Hemiptera, the Lepidoptera, the Diptera, and the Hymenoptera. 



The jointed beak of the Hemiptera consists of a troughlike 

 labium partially covered above at its base by the labrum. 

 Within this trough the elongated maxillae and mandibles are 

 ensheathed. 



The long, coiled proboscis of the Lepidoptera is formed of 

 parts of the maxillae, while the labrum, mandibles, and labium 

 are greatly reduced or wanting. 



The female mosquito has a piercing type of mouth parts. 

 The labrum and epipharynx are fused and with the hypopharynx 

 form the food channel. The linear mandibles and maxillae are 

 used in puncturing the skin of the victim, while the labium 

 forms a sheath for the other mouth parts. 



Suctorial and mandibulate functions are both performed by 

 the mouth parts of the honeybee. The mandibles are used for 



