690 MR. P. R. AWATI ON THE MECHANISM 
This sinking in, or involution, of these structures has been 
brought about by the greater development (overgrowth) of the 
clypeus and the labrum. The retort-shaped organs are nothing 
but the swollen bases of these stylets. Not only are these organs 
found in the Homoptera but they are also found in the Hetero- 
ptera; though in the latter they are not so prominent as in 
the former, owing to their degeneration. In the Homoptera 
they have been recently demonstrated by Davidson (9). 
The fact that both the stylets sink into the head owing to the 
overgrowth of the clypeus, has been recently shown by Heymons. 
The mandibles and the maxille, then, do not disappear but 
persist in the later stages, though in different positions. ‘‘ Man- 
diblen und Maxillenladen ziehen sich bei den Rhynchoten 
in tiefe taschenformigen Hohlungen zuruck und scheiden die 
chitinosen Stechborsten aus.” Thus writes Heymons. The 
retort-shaped organs of Mecznikow, in the Homoptera, are the 
‘‘taschenformigen Hohlungen” of Heymons, in the Hemiptera in 
general. 
Prof. J. B. Smith (45) was the first to call attention to the 
fact that the maxillary stylets form but a part of the first maxille. 
Each maxilla consists of two parts: the maxillary sclerite or 
segment, and the maxillary stylet; but he was not able to 
identify them separately. 
It was not until Heymons had published his “ Beitrige zur 
Morphologie und Entwickelungsgeschichte der Rhynchoten” (20), 
that the relation between the maxillary segment or plate and 
the maxillary stylet was clearly understood. According to Smith 
the two pairs of stylets with the lateral (maxillary) sclerites 
posterior to the mandibles, together with the proboscis, represent 
the first maxille ; the stylets representing the lacinia and stipes ; 
the sclerite representing the palpus and the proboscis. He had 
made both the stylets arise from the same place. He is apparently 
led astray in his interpretation by a faulty dissection, which was 
shown to be the case by Marlatt (34). 
Heymons is the first writer to explain clearly the homologies 
of the first maxille. He has studied their development in certain 
Hemipterous (both Heteropterousand Homopterous) forms. He 
has clearly demonstrated that a maxilla arises as a single structure, 
but that soon after it is divided into two parts :—(i.) a median 
piece, or maxillenlade; (ii.) a lateral piece, maxillenhécker, or 
maxillary plate (segment). 
(i.) Lhe maaxillenlade.—This becomes elongated and transformed 
into a long tapering stylet, two of which (one from either side) 
combine together to form two tubes, one for suction of the sap, 
and the other for ejection of the saliva. 
(ii.) The maxillary plate—This represents the stem of the 
maxilla (cardo and stipes) of the biting insects. It forms the 
antero-lateral piece of the head-wall, and has therefore nothing 
to do with the mouth-parts proper—except in so far as it forms 
the support to the protractor muscles of the maxille. 
