PHYLUM ARTHROPODA. INSECTS 



251 



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•Abdomen 



Prothorax 

 Compound eye 



Mandible 

 Gena 



Mesothorax Metafhorax Hindwing 



Forewing 

 Tympanum 



Ovipositor 

 Cercus 



Labrum 

 "-Clypeus 

 Antenna 



Figure 131. External features of the grasshopper, a good example of a generalized insect. 

 This is a female. 



face fitted for grinding. Behind the mandi- 

 bles are a pair of maxillae, consisting of sev- 

 eral parts and with sensory palps at the sides. 

 The labium or lower lip has slender palps 

 at the sides. The labium of insects appears 

 to have evolved from the lateral union of 

 two appendages resembling the biramous 

 limbs of a crustacean. The maxillae obvi- 

 ously have also arisen from this type of ap- 

 pendage. The labrum and labium serve to 

 hold food between the mandibles and maxil- 

 lae, which move laterally and grind it. The 

 maxillary and labial palpi are supplied with 

 sense organs that probably serve to distin- 

 guish different kinds of food. 



Ocellus 



The simple eye (Fig. 133) consists of a 

 group of visual cells, the retina, pigment, 

 and a transparent lens, a modification of the 

 cuticle. 



Compound eye 



The compound eye (Fig. 132) is covered 

 by a transparent part of the cuticle, the 

 cornea, which is divided into a large num- 

 ber of hexagonal pieces, the facets. Each 

 facet is the outer end of a unit known as an 

 ommatidium. Such a structure gives mosaic 

 vision as described in the crayfish (p. 209). 

 Some insects, possibly the grasshopper, are 

 able to distinguish colors. 



Antennae 



These are threadlike in form and consist 

 of many segments. Tactile hairs and olfac- 

 tory pits are present on them; and this con- 

 dition, combined with the ability of the 

 insect to move them about, makes them 

 efficient sense organs. 



Thorax 



This portion of the body is separated from 

 the head and abdomen by flexible joints and 



