ii8 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



the depression between the two ridges in the median line of the frons. For the 

 structure of the ocelli see Hegner (p. 245). Draw a front view of the head. 



The antennae are the first pair of head appendages. They are jointed fila- 

 mentous structures, springing from depressions in the upper part of the frons. 

 The antennae are very important olfactory and tactile appendages. Remove 

 an antenna and examine with the low power, noting the sensory hairs upon it. 



With the forceps, remove the clypeus and labrum in one piece, and examine 

 the under surface. This forms the roof of the mouth and bears a central club- 

 shaped elevation, the epipharynx, probably of sensory function. 



The removal of the labrum exposes the three pairs of mouth parts. The first 

 of these, constituting the second pair of head appendages, is the mandibles, very 

 hard brown organs with toothed inner edges. Remove these and draw one 

 under a hand lens. The next pair of mouth parts, the third pair of head append- 

 ages, are the maxillae (first maxillae). They are lateral and each has a con- 

 spicuous process, the palp. Remove a maxilla complete and study it with the 

 hand lens. It consists of a basal portion, composed of two segments, a lower 

 car do and an upper stipes, from which springs three processes. The inner one 

 is a curved, toothed blade, the lacinia; the middle one an oval plate, the galea, 

 composed of two joints; and the outer, a slender jointed process, the maxillary 

 palp, supported by a small basal joint, the palpifer. Draw the maxilla. It is 

 probable that the palp is the endopod, and the remainder of the appendage the 

 protopod, exopod being absent. 



The last pair of mouth parts, the fourth pair of head appendages, is the labium, 

 or lower lip, lying below the mouth in the median line. It is composed of two 

 maxillae (the second maxillae) partially fused in the median line. Attached 

 to the labium and projecting inward from it to form the floor of the mouth cavity 

 is an elevation, the hypopharynx, which serves as an organ of taste. Remove 

 the labium completely and identify the following parts with the hand lens: the 

 basal crescentic segment, the submentum; the next single piece, the mentum; 

 the paired labial palps, springing from the sides of the mentum through a small 

 joint, the palpifer; and the paired flat median plates, the ligulae. Draw the 

 labium. 



Since the head bears four pairs of appendages it must consist of at least four 

 segments. Investigation of insect embryos has shown that there is another 

 segment in front of the one bearing the antennae, and still another between the 

 antennary and mandibular segments. There are thus six segments in the insect 

 head. The antennae of the insects correspond to the antennules of the lobster; 

 while the segment and the appendages which correspond to the antennae of the 

 lobster are lost in the adult insect. 



b) The thorax and its appendages: The thorax is composed of three stout 

 segments, called the prothorax, mesothorax, and metathorax, respectively, begin- 

 ning at the anterior end. The tergum, or no turn, called the pronotum, of the 



