3;j^Q MR. J. DAVIDSON ON THE MOUTH-PARTS AND 



the close relationship that exists between the trophi of Heteroptera and 

 Homoptera. 



To supplement this brief review of the literature I shall indicate as far as 

 possible the relationships which the mouth-parts herein described bear to 

 those studied in other Hemiptera. 



The salivary pump is the " Wanzenspritze " noted by Landois (1868) in 

 Citnex lectularius. A few years later it was described by Mayer (1874) 

 in Pyrrhocoris aptems, and since then, from time to time, by many authors 

 in different species of Hemiptera, including Aphids and Coccids, Muir and 

 Kershaw (1911 &) consider that the salivary pump arises as a modified 

 development of the salivary duct. 



The morpholooy of the sclerites forming the head is a much debated 

 question. Owing to the deflexed position of the head in Homoptera the 

 distal part forms a freely articulating chitinous capsule which sujiports the 

 trophi. This, which is the Vorderkopf of many German authors, I propose 

 to call the fore-liead. Meek (1903) has described the fore-head in Cicada as 

 made up of the frons, clypeus, labrum, epipharynx, and the mandibular and 

 maxillary sclerites. The tapering upper lip, which I call the labrum, appears 

 to be the part referred to by Meek as the prolongation of the epipharynx ; 

 the labrum he considers as being considerably reduced. 



This view is also held by Muir and Kershaw (1911). It will be seen later 

 that I retain the name epipharynx for the specially thick chitinous roof of 

 the pharyngeal duct, which also forms the floor of the part I call the clypeo- 



labrum. 



The structure I refer to as the clypeus corresponds in position to the 

 frons of Meek in Cicada. A slight transverse depression divides it off from 

 the clypeo-labrum, but internally it is defined by the presence of the 

 pharyngeal struts, which are really thickened parts of the clypeal folds. Tlie 

 clypeus supports the divaricator muscles of the pharynx. 



There is some doubt as to the parts which make up the labium. Geise 

 (1883) considers it as the lower lip. L6on (1887) described rudimentary 

 labial palps in Hydrocores ; but Heymons (1899) found, by studying the 

 development in the embryo, that the proboscis is formed by the fusion of a 

 simple pair of appendages (2nd maxilloe), which bear no trace of palps. He 

 considers these latter structures arise as secondary developments. Muir and 

 Kershaw (1911) have shown that the proboscis is formed by the fusion of 

 the simple 2nd maxillas, and afterwards becomes secondarily divided into 

 segments ; they found no clue to the missing palps. 



The morphology of the stylets is a much debated point. Originally it 

 was thought that they represented the maxillsB and mandibles. Smith (1892) 

 drew attention to the fact that certain sclerites, which formed part of the 

 chitinous capsule of i\\e fore-liead, were really associated during development 

 with the stylets, so that the stylets do not really represent the complete 

 mandibles and maxillae. 



