September io, 1891] 



NATURE 



457 



SOME DIFFICULTIES IN THE LIFE OF 

 AQUA TIC INSECTS} 



■\^E understand insects to be animals of small size, furnished 

 with a hard skin and six legs, breathing by branched air- 

 tubes, and commonly provided in the adult condition with 

 wings. The animals thus organized are pre-eminently a 

 dominant group, as is shown by the vast number of the species 

 and individuals, their universal distribution, and their various 

 habitat. 



The insect type, like some fruitful inventions of man — paper 

 or lithography, for instance— has proved so successful that it has 

 been found profitable to adapt it to countless distinct purposes. 

 I propose to consider one only of its infinitely varied adapta- 

 tions, viz. its adaptation to aquatic life. 



There are insects which run upon the earth, insects which fly 

 in the air, and insects which swim in the water. The same 

 might be said of three other classes of animals — the three 

 highest — viz. mammals, birds, and reptiles. But insects surpass 

 all other classes of animals in the variety of their modes of exist- 

 ence. Owing to their small size and hard skin, they can burrow 

 into the earth, into the wood of trees, or into the bodies of other 

 animals. There are some insects which can live in the water, 

 not as the mammal, bird, or reptile does, coming up from time 

 to time to breathe, but constantly immersed, like a fish. This 

 is the more remarkable because insects are, as a class, air- 

 breathers. Air-tubes or tracheae, branching tubes, whose walls 

 are stiffened by spiral threads, supply all the tissues of the body 

 vyith air. That such an animal should be hatched in water, and 

 live almost the whole of its life immersed, a thing which 

 actually happens to many insects, is a matter for surprise, and 

 implies many modifications of structure, affecting all parts of the 

 body. 



The adaptation of insects to aquatic conditions seems to have 

 been brought about at different times, and for a variety of dis- 

 tinct purposes. Many Dipterous larvae burrow in the earth. 

 Some of thesp frequent the damp earth in the neighbourhood of 

 streams ; othtis are found in earth so soaked with water that it 

 might almost be called mud, though they breathe by occasionally 

 taking in atmospheric air. In yet more specialized members of 

 the same order we find that the larva inhabits the mud at the 

 bottom of the stream, and depends for its re-piration entirely 

 upon oxygen dissolved in the water. The motive is usually that 

 the larva may get access to the decaying vegetable matter found 

 in slow streams, but so ne of these larvse have carnivorous pro- 

 pensities. 



Other insects merely dive into the water, coming up from 

 time to time to breathe, or skate upon the surface. 



Nearly every order of insects contains aquatic forms, and the 

 total number of such forms is very large. I believe that all are 

 modifications of terrestrial types, and it is probable that 

 members of different families have often betaken themselves to 

 the water independently of one another. 



The difficulties which aquatic insects have to encounter begin 

 with the egg. It is in most cases convenient that the egg should 

 be laid in water, though this is not indispensable, and the 

 winged, air-breathing fly is, as a rule, ill fitted for entering 

 water. Some insect-eggs hatch if they are merely scattered, 

 like grains of sand, over the bottom of a stream, but others must 

 be laid at the surface of the water, where they can gain a 

 sufficient supply of oxygen. If the water is stagnant, it will 

 suffice if the eggs are buoyant, like those which compose the 

 egg-raft of the gnat, but this plan would hardly answer in 

 running streams, which would carry light, floating eggs to great 

 distances, or even sweep them out to sea. Moreover, floating 

 eggs are exposed to the attacks of hungry creatures of various 

 kinds, such as birds or predatory insect larvae. These difficulties 

 have been met in the cases of a number of insects by laying the 

 eggs in chains or strings, and mooring them at the surface of 

 the water. The eggs are invested by a gelatinous envelope, 

 which swells out, the moment it reaches the water, into an 

 abundant, transparent mucilage. This mucilage answers more 

 than one purpose. In the first place it makes the eggs so 

 slippery that birds or insects cannot grasp them. It also spaces 

 the eggs, and enables each to get its fair share of air and sun- 

 light. The gelatinous substance appears to possess some anti- 

 septic property, which prevents water-moulds from attacking the 



' Evening Discourse, delivered before the British Association, Cardiff 

 1891, by L. C. Miall, Professor of Biology in the Ycrkshire College. ' ' 



NO. 114 1, VOL. 44] 



eggs ; for, long after the eggs have hatched out, the transparent 

 envelope remains unchanged. The eggs of the frog, which are 

 laid in the stagnant water of ditches or ponds, float free at the 

 surface, and do not require to be moored. The eggs of many 

 snails are laid in the form of 'an adhesive band, which holds 

 firmly to the stem or leaf of an aquatic plant. Some insects, 

 too, lay their eggs in the form of an adhesive band. In other 

 cases the egg-chain is moored to the bank by a slender cord. 



The common two-winged fly, Chironomus, lays its eggs in 

 transparent cylindrical ropes, which float on the surface of the 

 water. During the summer months these egg-ropes, which are 

 nearly an inch in length, may readily be found on the edges of 

 a stone fountain in a garden, or in a water-trough by the side 

 of the road. The eggs are arranged upon the outside of the 

 rope in loops, which bend to right and left alternately, forming 

 sinuous lines upon the surface. Each egg-rope is moored to the 

 bank by a thread, which passes through the middle of the rope 

 in a series of loops, and then returns in as many reversed and 

 overlapping loops, so as to give the appearance of a lock-stitch. 

 The threi^d s so tough that it can be drawn out straight with a 

 needle without breaking. If the eg^rope is dipped into boil- 

 ing water, the threads become apparent, but in the natural 

 state they are in isible, owing to their transparency. The 

 mucilage is held together by the threads interwoven with the 

 mucilage. The loops can be straightened without injury until 

 the length of the rope is almost doubled. If stretched beyond 

 i this point the threads become strained, and do not recover their 

 I original shape when released. By means of these threads, 

 firmly interwoven with the mucilage of the egg-rope, the whole 

 I mass of many hundreds of eggs is firmly moored, yet so moored 

 I that it floats without strain, and rises or falls with the stream. 

 [ The eggs get all the sun and air which they require, and neither 

 predatory insects, nor birds, nor water-moulds, nor rushing 

 { currents of water, can injure them. 



I The egg.s of the caddis- fly are laid in larger ropes, which, in 

 j some species, are very beautiful objects, owing to the grass- 

 green colour of the eggs. The egg-raft of the gnat, which has 

 often been described, is well suited to flotation in stagnant 

 water, and is freely exposed to the air, a point of unusual im- 

 portance in the case of an insect which in all stages of growth 

 seems to need the most efficient means of respiration, and whose 

 eggs are usually laid in water of very doubtful purity. The 

 lower or submerged end of each egg opens by a lid, and through 

 this opening the larva at length escapes. 



The eggs of water-haunting insects are in many ways particu- 

 larly well suited for the study of development. The eggs of 

 Chironomus, for instance, can always be procured during the 

 summer months. They are so transparent as to admit of ex- 

 amination under high powers of the microscope as living objects, 

 and as they require no sort of preparation, they may be replaced 

 in the water after each examination to continue their develop- 

 ment. This saves all trouble in determining the succession of 

 the different stages— a point which usually presents difficulties 

 to the embryologist. The whole development of the egg of 

 Chironomus is completed in a few days (three to six, according 

 to temperature), and it is therefore an easy matter to follow the 

 process throughout with the help of three or four chains of eggs. 

 When the larvae are hatched, and escape into the water, new 

 difficulties arise. Some have to seek their food at the surface 

 of the water, and must yet be always immersed, others live upon 

 food which is only to be found in rapid streams, and these run 

 serious risk of being swept away by the rush of water. All need 

 at least a moderate supply of oxygen, which has either to be 

 drawn from the air at the surface, or extracted from the water 

 by special organs. The difficulty of breathing is, of course, 

 greatly increased when the larva seeks its food at the bottom of 

 foul streams, as is the case with certain Diptera. The larva of 

 Chironomus, for example, feeds upon vegetable matter, often in 

 a state of decay, which is obtained from the mud at the bottom 

 of slow streams, and in this mud the larva makes burrows for 

 itself, cementing together all sorts of materials by the secretion 

 of its salivary glands, drawn out into fine silken threads. The 

 burrows in which the larva lives furnish an important defence 

 against fishes and other enemies, but they still further increase 

 the difficulty of procuring a supply of air. Hence, the larva 

 frequently quits its burrow, especially by night, and swims 

 towards the surface. At these times it loops its body to and 

 fro with a kind of lashing movement, and is thus enabled to 

 advance and rise in the water. From the well-aerated water at 

 the surface of the stream it procures a free supply of oxygen, 



