Oct. 5, 1888.] 



SCIENTIFIC NEWS. 



365 



WATER EGGS OF INSECTS. 



"\A/"E doubt if there be any equally common object so 

 full of instruction to the young biologist as a string 

 of the eggs of Chironomus. The flies which bear this 

 name are often called midges. They might be summarily 

 described as stingless gnats, which hover in clouds over 

 water in still and warm weather. Their earlier stages 

 are passed in water, but the fly breathes air. 



At the time of egg-laying the female Chironomus ap- 

 proaches the water, and lays in it a chain of glutinous 

 eggs. The viscid secretion immediately absorbs water, 

 and swells to many times its original bulk. It then 

 appears as a flexible, transparent cylinder, about three- 

 quarters of an inch long, and perhaps one-tenth of an 

 inch thick. Near the surface of the cylinder are embedded 

 hundreds of yellow eggs, just visible to the eye as oval 

 specks in a clear field. The cylinder ends in a sticky 

 thread, by which the whole mass is moored to a stone or 

 some similar object. 



The naturalist who has a fountain in his garden may 

 expect to find these chains of eggs almost constantly dur- 

 ing the summer months. In brooks and ponds the eggs 

 are no doubt equally common, but much harder to find. 



For the study of the development of insects, no sub- 

 ject can be more advantageous. The eggs are almost 

 perfectly transparent, and may be examined alive with 

 the greatest ease, being afterwards returned to the water 

 to continue their growth. A low power of the microscope, 

 a live-box, and a little saucer of water are the only 

 appliances requisite. The development occupies three 

 days, and the eggs should be examined, whenever pos- 

 sible, at intervals of three hours. To study with profit, 

 drawings to scale should be made with care and regu- 

 larity. 



In newly-laid eggs nearly the whole space is occupied 

 by a granular yolk with many oil globules. At one end, 

 afterwards the hinder end, the peculiar objects known as 

 the polar cells may be seen. Early on the first day of 

 hatching, a single layer of transparent cells is seen to 

 invest the yolk, and within this a second layer rapidly 

 forms. From the outer layer (epiblast) are developed 

 the skin, nervous system, and sense-organs, while the 

 inner layer (hypoblast) gives rise to the epithelium of 

 the primitive alimentary canal. 



Towards the end of the first day the yolk has altered 

 its shape. A narrow extension runs out into the region 

 of the future head. Another tongue-like process passes 

 towards the transparent egg-shell, and opens upon the 

 back of the insect, which is as yet in a most incomplete 

 state. The top and side-views of the embryo are now 

 for the first time materially different from each other. 

 Two or three segments are indicated on the ventral sur- 

 face. By the middle of the second day a considerable 

 number of segments (body-rings) have formed. The 

 dorsal wall is still incomplete. Paired appendages, the 

 future antennas, mandibles and maxillae, are now to be 

 made out in the region of the head. The polar cells, 

 which have a singular history, have been taken into the 

 body, but can still be made out. They ultimately under- 

 go conversion into reproduclive organs. By the morning 

 of the third day all the segments are formed. The eye- 

 spots are conspicuous. A folding-in of the skin from the 

 head end has produced a long and narrow channel which 

 leads nearly to the yolk. A similar folding-in at the 

 opposite end of the body is in progress. The two new 

 lengths of alimentary canal thus formed constitute the 

 future oesophagus and intestine with their appendages. 



Uneasy movements are now to be seen, and a sort of 

 peristaltic action of the intestine sets in during the early 

 part of the third day. The body is by this time out- 

 wardly completed, and the organisation of the internal 

 organs proceeds with great rapidity, every hour producing 

 visible changes. At the end of about seventy-two hours 

 the egg-shell contains a transparent little larva, which 

 performs restless movements, and soon escapes from its 

 prison. After a few days the larva acquires the red 

 colour which has suggested the popular name of " blood- 

 worm." Its subsequent history is deeply interesting, 

 but we shall not pursue it at present. 



Many other small flies lay eggs in water after the 

 same fashion. The string of eggs of the caddis-fly is 

 much larger than that of Chironomus, and the eggs are 

 at first of a brilliant chlorophyll green colour. A curious 

 analogy exists between the insect-eggs just described and 

 the spawn of the frog or toad. In both of these very dis- 

 similar groups of animals the some artifice is used. The 

 oviduct secretes a gelatinous substance, which is poured 

 out upon the eggs, binding them together into a sort of 

 rope, which is small enough while still in the body of the 

 parent, but swells greatly when passed into the water. 

 The copious mass in which the eggs thus come to be 

 imbedded serves more than one purpose. It is slippery, 

 and cannot be seized by beak or claws — whether of bird 

 or rapacious insect. Secondly, it is transparent, and 

 thus serves to space the eggs widely, allowing light to 

 pass freely to each. The influence of light upon the 

 rate of development is very marked. Take any of the 

 eggs named (Frog, Caddis, or Chironomus), divide into 

 two parcels, place both in similar vessels, with the same 

 supply of air and warmth, but keep one set in the dark 

 and the other in the light. The eggs kept in the dark 

 will be found to get on slowly, and a considerable pro- 

 portion will not be hatched at all. Thus we see that the 

 peculiar requirements of development under water have 

 been independently met in precisely the same way by 

 animals too remote zoologically to admit the supposition 

 of a common tradition. 



THE DISTRIBUTION OF COLOURS IN 

 ANIMALS. 



SIGNOR L. CAMERANO, after a prolonged study 

 of this question, has laid before the Turin 

 Academy of Sciences certain results which have scarcely 

 received the amount of notice which they merit, and we 

 accordingly submit them to our readers. 



He arranges the colours of animals in the following 

 order as regards frequency of occurrence : — 1, brown ; 

 2, black; 3,- yellow, grey and white; 4, red; 5, green ; 

 6, blue; and 7, violet. 



These colours, however, are not uniformly distri- 

 buted in the main groups of the animal world. Black, 

 brown, and grey are relatively more abundant among 

 vertebrate animals than among the arthropods. Red and 

 yellow, on the contrary, are more frequent among the 

 non-vertebrated animals. Green is common among the 

 lower forms, with the exception of the molluscs, but 

 it is also fairly abundant among the vertebrata, always 

 excepting the mammalia. Violet and blue, the former 

 especially, are the rarest colours. White is very irregu- 

 larly distributed, but it is most common among dwellers 

 in the waters. 



The colours of animals are, on the whole, directly 

 related to the medium which they inhabit. Parasitic 



