THE INTERNAL STRUCTURE OF ANTS. 65 



Chapter XXYIII, the auditory and tactile sensations of insects are not 

 sharply distinguishable. 



The Campaniform Sensillae. These problematic organs have a very 

 simple structure, consisting of a thin, bell- or umbrella-shaped piece of 

 the chitinous cuticle forming the floor of a cavity in the much thicker, 

 undifferentiated chitinous layer of the integument (Fig. 31, C-H). 

 This cavity is narrowed externally and usually, but not always, opens 

 on the surface by means of a small pore (/>). A very delicate nerve 

 ( // ) terminates in the middle of the umbrella on its concave inner sur- 

 face. Organs of this description are found in various parts of the 

 insect body in the borders of the mandibles, at the bases of the wing 

 membranes, in the balancers of Diptera and in the trochanters and 

 bases of the femora. They have been found in these joints of the legs 

 and in the mandibles of ants by Janet (1904). In the mandibles occur 

 also other organs which may represent modifications of the campani- 

 form sensilke, although it seems more natural to refer them to the 

 ampullaceous type. The function of the campaniform sensillas is quite 

 unknown. Morphologically, according to Berlese, they may be derived 

 from a simple protaesthesis, in which the glandular element is lacking 

 and only the chitinogenous and nervous elements are present. 



The Lateral Eyes. Each of the lateral, or compound, eyes, consists 

 of a closely aggregated mass of sensillae, which are usually called 

 ommatidia and consist of the three elements of the typical prottesthesis : 

 hypodermal cells, which secrete the cornea ( facet ) , gland-cells, which 

 secrete the crystalline cone and nerve-cells known as retinulse. There 

 seem to be no comparative studies on the minute structure of the 

 ommatidia in ants. It is probable that they differ but little from those 

 of other insects. The great differences in size in the eyes of the dif- 

 ferent species and castes and their almost universal reduction or degen- 

 eration in the workers, induced Forel (1874) to investigate the size and 

 numbers of facets, which, of course, represent the ommatidia. He 

 found the size of the facets varying but little in the different species. 

 The smallest were seen in the male of Crcinastogastcr sordidnla, the 

 largest in the worker of Messor barbarns, but the latter were only twice 

 as broad as the former. The number of facets is extremelv variable 



w> 



from i in the worker of Poncra punctatissima, to 1,200 in the male of 

 Formica pratcnsis. -The variation in number of the different castes of 

 the same species is also very striking. Thus, in Tapinoma crraticnm, 

 the worker has 100, the female 2f>o, the male 400 ommatidia in each 

 eye; in Formica pratensis. the corresponding numbers are 600, 830 and 

 1,200, and in Snlenopsis fttga.r 6-9, 200. 400. A similar study of cer- 

 tain tropical ants (c. g., Eciton, Doryhts, Ponerinae, etc.) would give 

 6 



