40 INSECTS. 



be composed of many hexagonal corneas. Make a draw- 

 ing of a portion of it as seen under the microscope. 

 Count the corneas in a certain visible space. Estimate 

 what part this space is of the whole corneal layer, and 

 calculate the number of corneas in the whole layer. 



II. The Thorax. Note that the thorax is composed of 

 three segments. 



1. The small first segment, to which the first pair of 

 legs is attached, is the prothorax. 



2. The large second segment, to which the second pair 

 of legs and the first pair of wings are attached, is the 

 mesothorax. 



3. The third segment, to which the third pair of legs 

 and the second pair of wings are attached, is the meta- 

 thorax. Owing to 'the consolidation of meso- and meta- 

 thorax, the boundary between these segments may not 

 be very plainly seen. 



Observe that the thorax is in cross section more or less 

 quadrangular, showing more or less flattened dorsal, lateral, 

 and ventral surfaces. The dorsal surface is called the 

 notum, each lateral surface a pleurum, and the ventral 

 surface the sternum. The convenience of this notation 

 will be seen in the ease with which, by using it, any part 

 of the thorax may be designated. The upper or lower 

 part of the prothorax may be referred to as pronotum or 

 prosternum ; corresponding parts of the metathorax, as 

 metanotum or metasternum, etc. 



Examine one of the legs. Note that it, too, is made up 

 of segments, which, like those of the body, have a hard 

 outer shell of chitin. Flex and extend, and study the 

 action of the joints. 



1. The first small segment, the one by which the leg 

 articulates with the body, is the coxa. 



2. The next is a very small quadrangular segment, not 

 always easily made out, the trochanter. 



