EEPORT ON THE PELAGIC HEMIPTERA. 25 



4 



not quite, perpendicular. The middle lobe of the face is at least twice as broad as the side 

 lobes, a little dilated at the apex, somewhat prominent and truncate. The side lobes are 

 sub-triangular and rather inconspicuous. The sides of the head are blunt and not margined. 

 On each side of the upper surface of the head are three more or less conspicuous black 

 points,' rising above the pubescence, one at the posterior angle near the eye, one situated 

 more inwardly, and in a line with the front of the eyes, and one near the inner angle of the 

 base of the antenniferous tubercles (PI. III. fig. 7). Under the microscope these spots are 

 seen to be tubercles destitute of the pubescence which covers the rest of the head. The 

 summit of each tubercle is concave with raised margins, and, situated within the rim and to 

 one side, is a smaller tubercle, bearing a short hair (PI. III. fig. 8). In two species 

 (wiillerstorffi and sericeus) the apex of these tubercles is from '02 mm. to '022 mm. in 

 diameter, and the length of the hair "06 mm. to "10 mm. On examining one of the tubercles 

 from the inside of the head it appears to be hollow, with the base of the hair protruding 

 (unless this is an optical illusion) for some way into the ca\dty. It is probable that these 

 tubercles are organs of one of the senses, jiossibly that of sight. The situation of the 

 posterior ones corresponds with that of the ocelli, with which the Hemiptera are frequently 

 provided, but when these are present in the Heteroptera they are never more than two in 

 number, n,nd true insects in the adult condition have very rarely, if ever, more than three, ^ 

 though there is reason to believe that the anterior ocellus when present really consists of 

 two ocelli coalesced. This anterior pair would correspond to the middle pair in Halohates, 

 and the anterior pair (in this genus) has disappeared in other insects (as has in some 

 cases the middle pair, and in others all the pairs), though still existing in some groups of 

 the Arthropoda. The nature of these tubercles seems to have been hitherto overlooked, 

 and it is much to be desired that naturalists who have the opportunity should examine 

 the structure in fresh specimens, and ascertain with what part (if any) of the nervous system 

 they are connected. 



The Eyes are large and prominent, situated at the hinder angles of the head, and ex- 

 tending for about one -third of their length behind it, resting on the sides of the thorax. 

 The orbit, especially above and below, and in a less degree in front, is provided with 

 several irregular series of long, strong hairs, curved at the tip, the hairs nearest the eye 

 being the largest. Viewed from above, the eye is semi-circular in outline, with the inner 

 edge slightly convex ; viewed from the side it is roundly oval ; viewed from below the 

 outline is similar to that from above. The facets of the eye are hexagonal. In iviiller- 

 storffi their diameter is about '035 mm. ; in sericeus about '025 mm. The outer free 

 periphery has (in two species) about 25 facets. 



The Antennce are attached to the inner anterior apex of conspicuous tubercles, which 

 are situated on the side of the head between the eyes and the apex of the face. These 



^ Some Aphides are said to have six. 



(ZOOL. CHALL. EXP. — PART XIX. — 1883.) T 4 



