Edward Stuart Russell, The Directiveness of organic activities, Cambridge, Cambridge University Press, 1945, pp. 18-19.
Repair through behaviour
Let us take now an example of a very differend kind, illustrating the restoration of structuro-functional norms through the behaviour of the animal as a whole, not through the co-ordinated activity of its cells.
The caddis larva Molanna, the behaviour of which has been studied with great care by Dembowski (1933), builds for itself a case of charateristics shape ans structure which is normal for it. The case is usually made of sand grains, though other materials may be used, and it consists of a conical tube widening towards the mouth, provided on either side with a flange or wing which projects considerably over the mouth.
If the larva is deprived ot its case, it will proceed to construct a new one of the normal pattern. It buries itself in the sand, back downards, and pushes a mass of sand grains towards its head, where it binds them into a loose bundle with silk produced from the labial glands. This form a sort of anchor or holdfast to which the larva clings by its anal hooks while it gets on with the job of making a case.It makes first a ring or section of tube round its body, searching about for material, picking it up and often rejecting what is unsuitable, fitting suitable pieces accurately into the growing mosaic and binding them with silk. By addition of material to its anterior edge the ring is gradually extended into a tube, and when it is long enough the larva cuts it free from the holdfast. It builds the wings by stretching at full length out of the tube, but as it always keeps its anal hooks inside the tube it cannot reach back far enough to complete the hinder part of the wings. It usually achieves the normal and typical shape by lengthening the tube in front and cutting it off behind.
Cutting away of an underwanted part may also occur in the following circumstances. A number of larvae are ejected from their cases, but given an opportunity to return to them; they do not sort out accurately each into its own case, and it happens that a small larva acquires a large case ; next morning it is found ti have cut a piece off the front end of the tube (Fig. 3 ,a), and next day a large part of the overhanging roo (b), thus adjusting the size of the case to its own requirements.
If constrained to build with insufficient material, the larva may utilise for construction material taken from the holdfast, using this up completely; material is used in accordance with the dominant need or drive, which is to build the case.
Demboswki carried out a large number of experiments on the repair of the case with interesting results. If the roof or part of it is removed, or if the roof is damaged, the missing parts are completely restored. Here reconstruction is simply a repetition of original construction, and a straightforward replacement of missing parts. If, however, the hind part of the case (up to 15% of its length) is removed, the larva does not replace it, but build on to the cas in front sometimes too little, sometimes far too much. If the posterior half of the case is cut away, it generally extends the case in front to more or less normal proportions; but it may reverse its direction in the tube, restore the missing part, and then turn back to the front end. In five cases out of the thirty-two studied, the larva permanently reverse dits direction, building a tube with wings and roof at the hinder end and making that functionally the front end. In one cases the larva made a roof at the hind end, then cut it off, turned back and extended the case in front.
If two-thirds of the case are removed from behind, the response is extraordinarily variable. Of fifty-four cases no two were restored in exactly the same way, but six main kinds of response may be distinguished. The larva may (1) build a complete new case, using the old piece simply as a holdfast and later cutting it off (2) restore normality by building on in front, (3) build on at the hind end, then turn, cut off roof and part of tube, but rebuild them, (4) build a new front end on the hinder part and inhabit it, while the original front is left intact, (5) build a new front end behind and trim. Off the original front, so that the polarity of the case is completely reversed, (6) build first in front, then cut off the new roof, reverse and build at hind end. In twelve cases out of the fifty-four polarity was more or less completely reversed.
The stimuls to restoration is in all cases the tactile perception of something missing, « The body of the larva is always in contact with the inside of the tube, and the animal can very well detect when anything is missing » ( p. 296). The animal will alive quite happily in a piece of hollow stem, which satisfies its contact needs, though i twill build a roof to it and then extend the tube somewhat.
It will be seen that the varied behaviour of the Molanna larva in response to damage to its case is directive towards the restoration of the normal state, and is elicited by lack of the normal contact relations. It is clear too that the same functional end-state, enabling normal life to continue, is reached by many different ways. Restoration of the normal structure of thecase is not always achieved; the larva may be left with two front doors, as in the instances of « polar heteromorphosis »; but often i twill cut away what is unwanted, and achieve a good approach to structural normality.
It is noteworthy also that in responding to these operational defects the larva has to deal with unusual contingencies, which are unlikely to have presented themselves in its individual or its racial history ; its power of effective response is therefor something fundamental and primordial, not to be accounted for by selection. Very often, an animal’s instinctive response is limited ans streotyped, adequate to the unusual (see Russell 1944). The plastic behaviour of Molanna serves to remind us that the specialisation of instinctive behaviour is by no means absolute or universal; it is more probably a secondary development, a restriction or limitation of the general power of adaptive response which all organisms possess in greater or less degree.
As Dembowsky very rightly points out, the processes of repair and reconstruction of the case, which the larva of Molanna effects through behaviour, show a stricking similarity to the phenomena of regeneration and restitution, which are due to cell activities. Points of ressemblance are as follows: (1) repair tends to take place in the direction of normal growth, as is shown especially in the replacement of anteriors defects ; (2) there may be, not a simple replacement, but a restitution of the case as a whole, when a large part of the hind end is removed; (3) polar heteromorphosis is of frequent occurrence, and also hyperregeneration; (4) the rate of restoration is proportional to the amount of damage; and (5) « morphallaxis » (see p.138) may occur. The last-named phenomenon is shown when, if the roofs is removed and no loose material is available, the larva constructs the missing part with fragments taken from the case, and when the larva building a new case withdraws material from the holdfast it it can find none around it.
The analogy betwen siple repair and wound-healing will not have escaped the reader; there is the same response to something missing, the same directive action towards the restoration of normality, the same elimination of want is unwanted.