COMPOUND EYES 107 



2. The Eyes and Light Perception 



The eyes, or photo-receptors of insects, vary greatly in 

 structure and degree of development. Almost every condition 

 of differentiation of these organs is met with among various 

 insects or their larvae. In their simplest phase the eyes consist of 

 single visual elements, or sensillai, sensitive mainly to variations 

 in light intensity. The most complex and highly developed 

 organs consist of aggregations of many thousands of sensillse, 

 and are capable of forming images of external objects. It has 

 been customary to group the different types of visual organs of 

 insects into two categories, viz., compound eyes and ocelli, and 

 certain features respecting them will be briefly considered. 



Compound Eyes. Compound eyes are composed of variable 

 numbers of individual sensillse, or ommatidia, closely apposed to 

 one another. In certain aberrant or degenerate forms only a few 

 ommatidia, or even a single ommatidium, may compose the eye, 

 but, as a rule, these elements are much more numerous, and 20,000 

 or more may be involved. The well-developed compound eye is 

 the highest type of visual organ found in insects. Notwithstanding 

 its complex structure, it is a less perfect optical instrument than 

 the vertebrate eye, for the reason that no general focussing 

 mechanism is known to be present. In the dragon-flies, Vigier 

 (1904) states that a means of accommodation, consisting of 

 myofibrils and tracheal branches, is present between the 

 ommatidia. By the contraction of these fibres compression of 

 the crystalline cones is presumed to result, while the action of 

 air pressure on the tracheae may allow of a certain amount of 

 elongation. This interesting observation has attracted but little 

 attention, and the subject is one requiring fuller exploration. In 

 the general absence of any focussing mechanism, it follows that 

 in the vast majority of insects the capability of compound eyes 

 for image formation can only be effective in producing a clear 

 impression within a very limited focal distance. The distinctness 

 of the image, therefore, depends partly upon the nearness of the 

 object concerned. This, however, is not the only factor, since 

 the number and size of the ommatidia are also important, for a 

 larger number of small ommatidia will, on the mosaic theory, 



