430 MISS A. M. EVANS ON THE STRUCTURE AND OCCURRENCE 



has kindly allowed me to work in the Entomological Laboratory of the 

 University of Manchester. 



I find it impossible to express how greatly I am indebted to Dr. A. D. 

 Imms for the invaluable assistance he has rendered in innumerable ways. 

 To li i in I owe the suggestion of the subject of investigation. I am glad to 

 have this opportunity of thanking Dr. Imms for the immense amount of 

 trouble he has taken in the furtherance of this work. 



Terminology. — Folsom in 1899 furnishes a complete list of the various 

 terms employed up to that date to designate the hypopharynx and maxillulse. 

 To these latter terms, proposed by Hansen, he objects on the grounds that 

 they imply unproved homology. He proposed to call the median element 

 of the compound " hypopharynx" the " lingua," and its lateral appendages, 

 the ' : superlingnse." His objection to Hansen's terms can no longer hold 

 after his own proof of the correctness of the assumption of this homology. 



Investigators on the morphology of the mouth-floor in Endopterygota 

 speak of the median portion as the hypopharynx and the lateral lobes pro- 

 jecting from or situated upon it as maxillulse. These terms possess the 

 advantage that they are free from the possibility of confusion with others 

 used to designate portions of the labium. They will for these reasons be 

 employed in the present paper. 



APTEEYGOTA. 



MachUidce. — The maxillulse are figured by Oudemans (35) under the name 

 " Paraglossae." They are described in some detail by Hansen (21. p. 31). 

 They are, he says, independent of the hypopharynx near the base of which 

 they are articulated to the skeleton of the head. He notes the "small single 

 jointed palp," and observes that each appendage shows, towards the tip, a ten- 

 dency to cleavage into two lobes. A detailed figure is given by Carpenter (6). 

 He refers to the " spicules, ridges and pits " which occur on their surface, 

 and states that the outer lobe of each is articulated with the basal sclerite. 

 They have, he says, " all the appearance of a reduced pair of jaws." 



Petrobius sp. — P. oudemansi, Carpenter (?). The headless remains of the 

 specimens were kindly identified as far as possible by Professor G. H. 

 Carpenter. Since these appendages are in Macldlidce in a less rudimentary 

 condition than in any of the other insects considered in this paper, their 

 structure in this species has been studied with some care. Observations 

 made on simple dissections were supplemented by examination of pre- 

 parations treated with potassic hydrate, and others, which in addition were 

 stained with acid fuchsin. 



The maxillulse, which lie between the mandibles and first maxillae, are seen, 

 after the removal of these pairs of jaws, to stand on the floor of the mouth, 

 above the base of the hypopharynx. Of this organ they obscure all but the 



