194 



BIOLOGY FOR BEGINNERS 



entirely of a light, tough and horny substance called chitin which 

 is usually protectively colored. The head, with its sense organs 

 and mouth parts, the thorax with its legs and wings, and the ab- 

 domen, with its vent and reproductive organs, are all readily 

 distinguished. 



Head 



Sense Organs. The antennae are- the most anterior append- 

 ages and, as usual, are many jointed and devoted to the senses 

 of touch and smell. There are two kinds of eyes, three simple 



ones located respectively at the base 

 of each antenna and on the ridge be- 

 tween them, and the large compound 

 eyes projecting from the sides of the 

 head and consisting of hundreds of 

 six-sided lenses. The shape, location, 

 and number of lenses in the eye seem 

 to adapt the insect for sight in several 

 directions at once, but the image 

 formed cannot be very sharp. 



Mouth Parts. The mouth parts of 

 the grasshopper are fitted for biting 

 and chewing hard foods and consist 

 of labrum, mandibles, maxillae, and 

 labium, named in order from the 

 anterior. Though the mouth parts of insects are very greatly 

 modified to suit all kinds of food, still these four sets of organs are 

 always present, so we must become familiar with their names and 

 appearance. 



The labrum is the two-lobed upper lip which fits over the strong, 

 toothed, horizontal jaws or mandibles. The pair of maxillae, or 

 accessory jaws, are next behind the mandibles. They aid in cutting 

 and holding food, and also have a sense organ, like a short antenna. 

 This is called a palpus (plural: palpi). Posterior to the maxillae 

 comes the labium or lower lip, a deeply two-lobed organ, also 

 provided with palpi, which aids in holding food between the jaws. 



FIG. 62. Part of coraeal 

 cuticle, showing facets, of the 

 .compound eye of a horsefly, 

 Therioplectes sp. (Photomicro- 

 graph by Mitchell; greatly 

 magnified.) From Kellogg. 



