THE EXTERNAL ANA TOMY OF INSECTS 47 



The hypopharynx. — The hypophdrynx 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 paragnatha 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 (18 16), 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 two 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 

 antennas 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 INIilne-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 ocular 

 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 tritocerebriim. And it has also been 

 shown that the protocerebrum innervates the co:npound eyes and ocelli; the 

 deutocerebnim, the antennae; and the tritocerebrum, the labrum. This 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 antennas and the mandibles doubt- 

 less belong. As Viallanes 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. 



Folsom ('00) in his work on the development of the mouth-parts of Anurida 

 described a pair of primary ganglia which he believed indicated the presence of a 

 segment between the mandibular and maxillary segments. He named the ap- 

 pendages of this segment the superlingiice; they are the paragnatha described above. 



The existence of the supposed ganglia indicating the presence of a super- 

 lingual segment has not been confirmed by other investigators and is no longer 

 maintained by Folsom. 



