THE EXTERNAL ANATOMY OF INSECTS 47 



The hypopharynx. The hypopharynx is usually a tongue-like 

 organ borne on the floor of the mouth cavity. This more simple form 

 of it is well-shown in the Orthoptera (Fig. 53). To the hypopharnyx 

 are articulated the maxillula? when they are present. The hypo- 

 pharynx is termed the lingua by some writers. 



d. THE SEGMENTS OF THE HEAD 



The determination of the number of segments in the head of an insect is a 

 problem that has been much discussed since the early days of entomology. The 

 first important step towards its solution was made by Savigny (1816), who sug- 

 gested that the movable appendages of the head were homodyanmous with legs. 

 This conclusion has been accepted by all; and as each segment in the body of an 

 insect bears only a single pair of appendages, there are at least four segments 

 in the head; i.e., the antennal, the mandibular, the maxillary, and the second 

 maxillary or labial. 



In more recent times workers on the embryology of insects have demonstrated 

 the presence of three additional segments. First, there has been found in the 

 embryos of many insects a pair of evanescent appendages situated between the. 

 antennae and the mandibles. These evidently correspond to the second antennae 

 of Crustacea, and indicate the presence of a second antennal segment in the head 

 of an insect. This conclusion is confirmed by a study of the development of the 

 nervous system. And in the Thysanura and Collembola vestiges of the second 

 antennae persist in the adults of certain members of these orders. 



Second, as the compound eyes are borne on movable stalks in certain Crusta- 

 cea, it was held by Milne-Edwards that they represent another pair of appendages; 

 but this view has not been generally accepted. It is not necessary, however, to 

 discuss whether the eyes represent appendages or not ; the existence of an ocalar 

 segment has been demonstrated by a study of the development of the nervous 

 system. 



It has been shown that the brain of an insect is formed from three pairs of 

 primary ganglia, which correspond to the three principal divisions of the brain, 

 the protecerebrum, the deutocerebrum, and the tritucerebrum. And it has also been 

 shown that the protocerebrum innervates the compound eyes and ocelli; the 

 deutocerebrum, the antennae; and the tritocerebrum, the labrum. 3Phis demon- 

 strates the existence of three premandibular segments: an ocular segment or 

 protocerebral segment, without appendages, unless the compound eyes repre- 

 sent them; an antennal or deutocerebral segment, bearing antennae; and a 

 second antennal or tritocerebral segment, of which the labrum is a part, and to 

 which the evanescent appendages between the antennae and the mandibles doubt- 

 less belong. As \ftallanes has shown that the tritocerebrum of Crustacea inner- 

 vates the second antennae, we are warranted in considering the tritocerebral 

 segment of insects to be the second antennal segment. 



Third, the presence of a pair of jaws, the maxillulae, between the mandibles 

 and the maxillae has been demonstrated in several widely separated insects. These 

 are doubtless the appendages of a segment, which is so reduced in most insects 

 that it has been overlooked until comparatively recently. Folsom ( f oo) in his 

 work on the development of the mouth-parts of A nurida demonstrated the exist- 

 ence 1 of the pair of primary ganglia pertaining to this segment. 



