cover-glass. Examine them with a compound microscope, 

 using a high power. The surface of each hair is marked 

 with numerous ridges, which resemble in appearance the 

 fine ridges on the skin of the palm of the human hand. In 

 the center of each hair, and extending nearly its en- 

 tire length, is a large tube. This is a trachea or air vessel. 

 Tracheae can be distinguished from other vessels by being 

 marked with transverse lines, which occur at regular and 

 very short intervals. The intimate structure of the tracheae 

 will be studied later ; but at this point the student should 

 become familiar with the appearance of tracheae, so that 

 when he dissects the insect he can readily distinguish them 

 from other vessels. Branching from this large central 

 trachea are numerous very small tracheae. Carefully trace 

 out the courses of the small tracheae and represent them in 

 your drawing. 



8. A spiracle. State number of spiracles and the position 

 of each pair. Describe a spiracle. Do you observe any differ- 

 ences between the different spiracles of this insect ? 



INTERNAL ANATOMY. 



Preliminary work. — Take from the chloral hydrate solu- 

 tion a specimen which was slit on the ventral surface. Im- 

 merse the insect in water, with its dorsal surface downwards. 

 With fine scissors extend the slit on the ventrimeson the 

 whole length of the thorax and abdomen ; in making this 

 slit cut through the wall of the body into the perivisceral 

 cavity ; the body-wall consists of two parts, the external 

 crust of the insect, and, entad of this a wall of muscles ; it 

 requires considerable care to cut into the perivisceral cavity 

 and not injure the viscera. Make, on each side, in that 

 part of the body-wall connecting the prothorax with the 

 head, an incision extending from the longitudinal incision 

 to the side of the body. 



