NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 49 



midline may be elevated to form an axial ridge (D) ; it may again 

 be more or less flattened (B, C, E), or reduced to a minute disc. 

 Its walls may be thick and densely chitinous, membranous, or so 

 thin as to be scarcely perceptible. The dome is sometimes freely ex- 

 posed on the surface of the body cuticula, but it is often protected by 

 chitinous outgrowths about it (A, E), or by being itself sunken into 

 the cuticula (D, F, G). In the sunken type, the cavity containing the 

 dome may open directly on the surface (D), or by means of a tubular 

 canal (F), while sometimes it appears to be entirely closed (G). In the 

 last case, however, as admitted by both Janet (1904) and Hochreuther 

 (1912), a pore to the exterior might be present, though escaping 

 detection in sections. 



The simplest of the campaniform organs are those in w^hich the 

 external part is reduced to a minute circular oval disc, which may be 

 situated at the surface of the body cuticula or sunken into a pit. 

 Those located at the surface or in shallow depressions of the body 

 wall are common on the wing bases, legs, and other parts of many 

 insects. They have been described particularly by Mclndoo under 

 the nanjfe of " olfactory organs " or " olfactory pores." Simple organs 

 of the sunken variety are described by Erhardt (1916) on the wing 

 bases of dragonflies, each consisting of a delicate, imperforate mem- 

 brane spanning the floor of a cuticular pit. The cuticular canal be- 

 neath the membrane contains the end process of a sense cell and the 

 distal part of an enveloping cell. 



Finaby, there should be mentioned here the organs found by Orlov 

 (1924) in the posterior part of the alimentary canal of the Lamelli- 

 corn larva, Oryctcs, since these organs apparently belong to the cam- 

 paniform group. Each consists, according to Orlov, of a delicate, 

 circular chitinous membrane (fig. 22, a) which may be slightly con- 

 vex or concave. Beneath the membrane is a large cell {CI), which 

 Orlov regards as a gland cell, and, beside this, a bipolar sense cell 

 {SCD, the distal process of which goes to the center of the mem- 

 brane. These organs, Orlov says, are similar in structure to sense 

 organs distributed over the body in Mclolontha larvae. Other beetle 

 larvae possess campaniform organs of the more usual type. Mcln- 

 doo (1918 a) describes simple campaniform organs ("olfactory 

 pores ") distributed over the head, antennae, mouth parts, thorax, 

 and legs of the larva of Allorhina (Scarabaeidee), and compound 

 organs on the terminal segments of the antennae. The compound or- 

 gans consist of groups of several hundred simple organs situated in 

 thin oval areas, or plates, of the antennal walls. 



The function of the campaniform organs is still a subject of specu- 

 lation. Some writers have suggested that the organs respond to vi- 



