THE EXO-SKELETON OF THE PROBOSCIS. 151 



regarded as a labium. It is articulated by syndesmosis with 

 the palpigerous scales, and there is no thyroid sclerite, but both a 

 furca and a discal sclerite are present. The furca is articulated 

 with an internal sclerite, probably the body of the furca invagi- 

 nated, and the discal sclerite is supported by two short, broad 

 paraphyses, which articulate with the edges of the labrum. 



Paired Lancets. One of the most marked characters in the 

 Blow-fly and all the Muscidae is the absence of paired lancets. 

 Two pairs of these are always present in the predaceous flies, 

 and a single rudimentary pair have been detected in some of 

 the Syrphidae. When two pairs of lancets are present, they are 

 similar to the two pairs of bristles found in the Hemiptera. 

 These have been named mandibles and maxillae in both orders, 

 but on very insufficient evidence. 



So far as I know, there is no dipterous or hemipterous 

 insect in which there are any traces of mandibles, and the parts 

 so named are always a part of the maxillae, and articulate with 

 the palpigerous scale. Dimmock [68, p. 28] identifies the parts 

 which Menzbier supposed to be mandibles in Syrphus, with 

 the maxillae. 



The Mouth Armature of the Pulicidse. The structure of the 

 mouth in the Fleas (Pulicidce) is of extreme interest in relation 

 to that of the Diptera, as it is manifestly intermediate in 

 character between the dipterous and the hemipterous mouth. 

 As long ago as 1749 Rosel* recognised the affinity of the Fleas 

 with the Diptera, a view also adopted by Straus Durckheim 

 and Oken,*f- and one which has been recently widely accepted. 

 Macleay+ perceived the intermediate relation of these insects 

 with the Hemiptera, on the one hand, and the Diptera on the 

 other. 



In the Fleas the palpiger (Fig. 33), which has frequently 

 been mistaken for a mandible, is a large triangular hollow blade 

 with trenchant edges. It supports a four-jointed palp near 

 its proximal margin. 



* Rosel, ' Insektenbelustigurigen.' 1749. 



f Oken, ' Naturgeschichte fur Schiilen,' p. 775. 1821. 



J Macleay, ' Horas Entomological,' i., p. 357. Lond., 1821. 



