THE OLFACTORY SENSE OF ORTHOPTERA 415 



ing on what families are compared; comparing the Acrididae, 

 Locustidae, and Gryllidae with one another, the variations are 

 small, but if these families are compared with the other three 

 families or if Blattidae is compared with Mantidae and Phasmi- 

 dae, the variations are large. The chief variation pertaining to 

 the genera, species, and sexes is in the number of pores present; 

 however, the pores may occasionally differ a little in external 

 structure. For example, those on the legs of the mole-cricket 

 (no. 43) are almost slit-shaped, while in the other genera they 

 are more or less eye-shaped. Twelve of the twenty males exam- 

 ined bear more pores than do their respective females, but, as a 

 rule, there is not much sexual variation in the number of pores. 

 For further details the reader is referred to table 1. 



Structure of pores in a grasshopper 



The preceding pages deal with the disposition of the olfactory 

 pores, and now a discussion of their anatomy will be given. 



a. External structure. When the superficial ends of the pores 

 are examined under a high-power lens with a strong transmitted 

 light, these organs appear as small bright spots, each of which 

 is surrounded by darker chitin, the pore border (figs. 20 and 52, 

 B), and by the pore wall (W). Sometimes the borders are 

 scarcely discernible, as for example, those on the abdominal seg- 

 ments (fig. 33) and those of the smallest isolated pores on the 

 legs (fig. 61) ; occasionally the borders and walls are very dark 

 (figs. 15, d, and 52) and the wall is wide; a few of the pores have 

 double borders (figs. 15,/, and 63), the outermost one being only 

 a little darker than the surrounding chitin and the innermost one 

 considerably darker; and occasionally the borders resemble some 

 of those on coleopterous and lepidopterous larvae (Mclndoo, 

 '19) in that they have radial streaks (figs. 15, g, and 53). The pore 

 wall may be round (fig. 55), oblong (fig. 51), but it is usually 

 eye-shaped (figs. 20 and 22) , and this is the first time for an eye- 

 shaped type to be described; this type is common to all the Or- 

 thoptera examined; however, on the legs of the mole-cricket, 

 the walls are almost slit-shaped, and in this respect somewhat 

 resemble the lyriform organs of spiders. 



