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J. DAVIDSON. 
composed of stout chitin, and is strongly dome-shaped on its 
anterior face. It freely articulates with the epicranium by 
means of thin, flexible chitin (figs. 23, 24), and is continued 
above the mouth as the pointed labrum or upper lip {Ibj-.). It 
has been termed the “ frons ” by some American writers 
(Ashmead, 1889, p. 185), and is the Vorderkopf ” of German 
authors. Bugnion (1911, p. 649) recognises the homology of 
this structure in Hemiptera with the clypeus of other 
insects. A slight transverse ridge occurs at the junction of 
the clypeus and labrum. 
The appendages of the head consist of the antennge and the 
buccal appendages, the latter comprising the labrum, a pair 
of mandibles, the maxillae, and the proboscis or beak (second 
maxillm). 
The buccal appendages of aphids are now generally accepted 
as homologous with the mouth parts of other insects as 
follows: the proboscis represents the labium, its anterior wall 
being continued beneath the mouth as a small underlip or 
hypopharynx ; the anterior pair of setae the mandibles; the 
posterior pair of setae the maxillae ii, the labrum being dis- 
tinct as a triangular prolongation of the clypeus. Smith 
(1892, p. 189) considers the setae represent the lacinia and 
stipes of the maxillae, developed as in the Diptera. This 
conception of the buccal appendages of Hemiptera was, 
however, proved to be erroneous by Marlatt (1895, p. 241), 
but Smith states in a later paper (1898, p. 176) that he 
is still of the opinion that the proboscis and setae are maxillary 
structures, and that no trace of mandibular structures occur 
in any present Hemipterous form. Marlatt (1895, pp. 241— 
249), as a result of careful investigations on the sti’ucture of 
the mouth parts of Cicada, shows that the mandibles in this 
insect are represented by two small sclerites and the anterior 
setae, to which the swollen bases of the latter structures are 
attached. Similarly the maxilla are represented by two 
narrower sclerites, to which the posterior setae are attached. 
In the lower families of Hemiptera Marlatt states that 
these sclerites are minute or obsolete. 
