C. Tomaszewski, « The principles of case building behaviour in Trichoptera » Giampaolo Moretti, Proceedings of the 3rd International Symposium on Trichoptera, 1980, La Haye, Dr W. Junk Publishers, 1981.
It is hypothesized that the caddis larva’s portable case is the result of tactile demands which originated from its soil-dwelling ancestors. The external stimulus that triggers the building activity is thought to be lack of touch.
When the wart-like protuberances on the 1st segment were compressed in an experimental research, building activity ceased. It is, therefore, suggested that these are the superior mechanoreceptors responsible for triggering building activity.
The portable case of the caddis larvae, although constructed almost entirely of foreign material, are an integral part of their bodies. Together they form a function unity, for without the case the larva could neither live nor develop. The problems of case building are, therefore, both phylogenic and ontogenic.
Ross (1964, 1967), Malicky (1973) and Tomaszewski (1973) investigated the evolutionary aspect of this problem and Tomaszewski, also, drew attention to its original causes. There is an ever increasing number of ontogenic papers which mainly deal with the methods of building, the plasticity and the repairing of the cases by the different species. However, they offer no answer to the question « Why does the larva build a case and what triggers the building mechanism ? » Some investigators have attempted an answer and, in 1965, Merrill demonstrated that removal of the anal hook and hair sensills of the 9th and 10th segments prolongs the case building activity, but that resection of the nerve cord between the 6th and 7th ganglia causes the activity to cease. From these experiments, Merrill concluded that the impulse emitted by the posterior ganglia pass to the supposed anterior control mechanism and stimulate building activity, whereas sensory contact of the anal hooks and hair sensills with the case inhibits the activity. These findings have been confirmed by Marstaller (1969), Hansell (1973, 1974a,b) and smart (1974). However, Marstaller doubtsMerrill’s interpretation, because when an additional cuti s made at the 4th and 5th ganglia, the building activity is not fully inhibited. Furthermore, Branch (1922) reported that the anal hooks of Limnephilidae are innervated by the 7th and 8th ganglia. Marstaller, therefore, considers that building activity is a function of the whole nervous system and his hypothesis is supported by the fact that when its posterior portion is estroyed by irradiation, the larva continues building. Smart’s (1974) experiments suggest that the case length is more dependent on the presence, or absence, of anal claws than of hair sensills. The only function of the anal hook, in my opinion, is that of attaching the larva to the case at the posterior inside and if the larve cannot do this, it cannot measure the lenght of the case and so continues building. The question on the causes of case building, therefore, remains open.
The investigations mentioned deal with internal which must be triggered by an external stimulus ; so one way well ask : « What is the external stimuls, how is it perceived and where are the superior receptors located ? »
The case structure is multifunctional ; it provides streamlining, ballast rigidity, buoyance, camouflage, internal water circulation and protection from predators and other dangers (Wiggins, 1977). So, maybe, the next question should be : « What is the original and most basic function of the case, that may have determined its building ? ». Hologenic data are used in an attempt to answer this question.
Tomaszewski (1973) hypothesizes that the process of adaptive evolution in the caddis larva can be visualized as a cycle of changes from terrestrial ancestors, through life in water to the larva of certain contemporary species that have been driven to live on the land. The cycle turns in time to generate a wave on the co-ordinate system. Fig. 1 depicts a single hologenetic turn ; on the first peak, the hypothetical soil-living ancestors, which probably stuck silk linings to their tunnels in the substrate, are seen. Research carried out by Marstaller (1969) and Tachet (1978) suggest that spinning and building behaviour are independent and, in this case, building is secondary to spinning behaviour. The passage from soil to lotic waters saw a new group of insects- the Trichoptera-arise. On the descending line two family branches-Hydropsychoidea and Rhyacophiloidae-evolved in cool waters (adapted from Ross, 1956) and building behaviour originated, while the evolution of the Limnephiloidae branch was associated with a passage into warm lenitic waters (adapted from Ross, 1956) and thence, in the larva of some species-Enoicyla pusilla (Burm.), Ironoquia parvula (Banks) (Flint, 1958) and Philocasca demita (Ross) (Anderson 1967)-secondarily, through lotic waters to the land. A new post-Trichoptera group of insects maye volve from these species in the future.
A large number of hair sensills are distributd in various patterns over the body of the larva. If lack of touch stimulates building activity, then there should be a superior touch receptor. I decided to investigate the abdominal wart-like protuberances on the first segment which, it is generally thought, have the function of attaching the anterior of the larva to the inside of the case. The function attributed to the mis questionable because they are too soft for this purpose. When the larva is out of the case they can be retracted, but not when it is inside. This suggest tthat they have another role, that of tactile receptors.
Materials and methods
The results are reported in Tables I and II. The numbers in columns 2 and 3 show burrowing and case buildings times on the first day after the case was removed ; they are given for comparison only. Column 5 lists non-building time in larva with compression girdles in place, which is, also, the time girdles were left on ; the differences between the specimens are, therefore, due to chance. The behavioue of the larva after the girdles were slipped off is recorded in column 6. It will be seen from columns 5 and 6 that while the girdle exerted pressure on the warts, there was no case building activity. The longest non-case-building times observed were 14 days in L. flavicornis and 15 days in L. borealis.
The case is a multipuporse structure, the basic function which determines its building is the satisfaction of the tactile demends and the external trigger mechanism is the lack of touch. The superior mechanoreceptors which receive the tactile information are the wart-like protuberance on the 1st segment, which, also, probably play a role in controlling case weight and length as the larva grows. Presumably, the shape of the case depends on the shape of the protuberances and their innervation system, but this is a subject for future research. By stretching out from the case with the help of its legs, while remaining hooked to the inside by the anal claws, the larva brings the abdominal protuberance into contact with the anterior rim and this, probably, the way the larva measures its case. Hansell (1974a) reported on stretchreceptors, but did not examine them in the Trichoptera.
It can, therefore, be concluded that the abdominal protuberance are responsible for transmitting the input of touch information on the stretch maximum to the central nervous system and that this dictates when building actyivity ceases, and that contact protuberances with the case wall satisfies tactile demands.