116 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



detected. As these bodies are well developed in the brain of the cockroach and locust and 

 probably nearly all adult winged insects, their apparent absence in Anophthalmus is noteworthy. 

 These sections establish, however, the fact of the complete absence of the optic ganglia as well 

 as optic nerve, and show how total is their atrophy. 



THE ANATOMY OF THE BRAIN OP ADELOPS. 

 (PI. XXIII, figs. 3. 3', 3", 3«, 3d). 



Transverse and horizontal sections* of the head of Adelops Mrtus, from Mammoth Cave, 

 reveal no traces of the optic ganglia or the optic nerves. The rudimentary eyes are situated 

 each on a lateral projection of the head (Fig. 3'), and are represented by a circular, convex area 

 equal in diameter to the thickness of the third antennal joint of the same insect. This area 

 is bounded by a well-defined, thickened margin.. It contains apparently not more than three 

 minute corneal facets, which are pale, not black. There is no black pigment, but the minute cor- 

 neal lenses or cones are surrounded within by a pale degenerate pigment mass. 



The brain, or procerebral lobes, on the other hand, appear, as shown by Figs. 3 a, 3 6, to be of 

 normal size and structure. 



There are no tactile hairs on or near the site of the eyes, though there are numerous rather 

 long ones extending from the top of the head around nearly to the eyes. 



It thus appears that Adelops must be blind, though the eyes exist in a rudimentary state. 



There are no optic ganglia or optic nerves. 



The number of facets is greatly reduced. 



There are only three, or nearly that number, of very rudimentary cones or corneal lenses. 



There is no black pigment, the lenses being partly enveloped by a pale rudimentary retina. 



THE BRAIN OP THE CAVE CRICKET. 



Ceuthophilus maculatus. — In this cricket, which lives above ground under sticks and stones, 

 the brain, as studied by gross dissection, is as usual in insects with well-developed eyes. The 

 procerebrum is of the normal size, but the optic ganglia are small compared with the procerebral 

 lobes, and the optic nerves are rather short and thick, the eyes being nearer together than in 

 Hadencecus. 



Hadencecus subterraneus. — In this cave cricket the eyes are but little smaller than in Ceu- 

 thophilus. The procerebrum is large and well developed, but the optic ganglia are small and of 

 the same relative size as in Ceuthophilus, while the optic nerves are a little longer and slenderer, 

 but no more than one would expect from the smaller eyes and their greater distance apart. 



IX— ORIGIN OF CAVE SPECIES AS AFFORDING FACTS FOR THE THEORY OF EVOLUTION. 



The main interest in the foregoing studies on cave life centers in the obvious bearing of the 

 facts upon the theory of descent. The conditions of existence in caverns, subterranaan streams, 

 and deep wells are so marked and unlike those which environ the great majority of organisms, 

 that their effects on the animals which have been able to adapt themselves to such conditions at 

 once arrest the attention of the observer. To such facts as are afforded by cave life, as well as 

 parasitism, the philosophic biologist naturally first turns for the basis of his inductions and 

 deductions as to the use and disuse of organs in inducing their atrophy. It is comparatively easy 

 to trace the effects of absence of light on animals belonging to genera, families, or orders in 

 which eyes are normally almost universally present. As we have seen in the foregoing list of 

 non-cavernicolous animals, the eyes are wanting from causes of the same nature as have induced 

 their absence in true cave animals. No animal or series of generations of animals, wholly or in 

 part, lose the organs of vision unless there is a physical appreciable cause for it. While we may 

 never be able to satisfactorily explain the loss of eyes in certain deep-sea animals from our 

 inability to personally penetrate to the abysses of the sea, we can explore caves at all times or 



* These sections were made for me by Mr. H. C. Bumpus. 



