The Breakdown of the Thoracic Gland in the Adult Insect, Rhodnius Prolixus

If an extra larval stage (6th-stage larva) is produced in Rhodnius by the implantation of the corpus allatum from a 4th-stage larva into the 5th-stage larva, it will usually moult again after feeding (Wigglesworth, 1936). Presumably, therefore, these: 6th-stage larvae retain their thoracic glands along with their other juvenile characters. This has been confirmed by dissection.

With the object of obtaining a series of intermediate stages, the corpus allatum from the 4th-stage larva was implanted into the abdomen of a number of 5th-stage larvae. The resulting forms ranged from normal adults with a small localized patch of larval cuticle over the site of the implant, through a series of more or less larval forms with enlarged and crumpled wings, to the usual 6th-stage type of larva (see Wigglesworth, 1936, pl. 5, figs. 12-16; and Wigglesworth, 1948, pl. 1, figs. 5 and 10-12). These intermediate forms were then fed. When it appeared (from the visible changes in the integument) that moulting had begun, they were dissected and the thoracic gland mounted. If they failed to moult they were fed again once or twice and finally dissected.

Those which approximate to the adult form (having only local patches of larval cuticle) show no sign of moulting and are found on dissection to have lost their thoracic glands.

Those approximating to the larval form (with varying degrees of enlargement of the wing lobes and of development of the genitalia) retain the thoracic gland cells, though in some the cells are reduced in number. After feeding, most of these insects moult. Moulting may proceed at the normal rate and in these the thoracic gland cells are numerous and active (Wigglesworth, 1952, fig. 6B); or moulting may be delayed and in some of these the thoracic gland cells appear to be reduced in number.

But in some of the larval forms moulting fails to occur in spite of full and repeated feeds of blood. One 6th-stage larva, produced by the implantation of the corpus allatum from an adult Periplaneta in October 1950, was fed repeatedly until April 1954 when it was dissected. In those larval forms which fail to moult the thoracic gland cells are present but show no signs of secretory activity. The number of these cells is sometimes reduced, but often appears normal. The failure to moult presumably results from some other defect in the endocrine system: the thoracic glands are not activated.

These experiments confirm the assumption that the juvenile hormone ensures the persistence of the thoracic glands. This is clearly effected by humoral means. It remains to be proved that their breakdown in the adult is also brought about by humoral means.

By reason of their great size and lobulation it is quite easy to recognize the nuclei of the thoracic gland cells after transplantation to the abdomen. In many of the experiments to be reported the glands (together with the slender lobe of the fat body with which they are associated) have been implanted immediately below the integument of the abdomen in another insect. At a later stage this region of the abdomen has been examined in whole mounts or in section.

If the glands are removed from an unfed 5th-stage larva and implanted into the abdomen of another 5th-stage larva one day after feeding they are still intact when this insect moults to become adult, but they then break down, synchronously with the gland cells of the host, during the next 24 hr.

The same results are obtained if the thoracic glands from an unfed 4th-stage larva are implanted into another 4th-stage larva soon after feeding. They remain intact until the host has undergone two moults and become adult; they then break down during the following day.

If a 4th-stage larva receives a thoracic gland implant from an unfed 5th-stage larva, is allowed to moult twice to become adult and is then dissected at once, the transplanted thoracic gland is still present. The juvenile hormone of the 4th-stage larva has led to its preservation from an extra instar. The gland then breaks down synchronously with the glands of the host.

Conversely, if the thoracic gland removed from an unfed 4th-stage larva is implanted into a recently fed 5th-stage larva it breaks down as soon as the host becomes adult. In this case the implanted gland has disappeared after going through four moulting stages instead of the usual five.

Since the thoracic gland, both in its normal position and after transplantation to the abdomen degenerates within 24-48 hr. after the adult moult, it is clear that this breakdown must be humorally determined.

We have seen that exposure to the juvenile hormone leads to the persistence of the thoracic glands. In the newly moulted adult the juvenile hormone is absent (Wigglesworth, 1948). It was therefore of interest to consider whether the glands can be preserved by exposure to the juvenile hormone at this stage.

The front half of the head was removed from 4th-stage larvae at 7 days after feeding, that is at a time when there is abundant juvenile hormone circulating in the blood, and they were then joined to newly moulted adults (through a small excision in the sternites) so that these received juvenile hormone in the circulating blood. The adults employed had moulted 2-6 hr. previously. They were dissected 5 days later and in all cases the thoracic glands had disappeared.

In other experiments the thoracic glands were dissected out from adults within less than 3 hr. after moulting and implanted into the abdomen of 4th-stage larvae at 7 days after feeding. The host was allowed to moult to the 5th stage and then dissected immediately. In none did the thoracic gland nuclei survive.

Clearly the thoracic gland of the newly moulted adult will not survive even in an environment containing abundant juvenile hormone. In order to ensure survival it must be exposed to the juvenile hormone during the preceding stage of growth leading up to moulting.

Will the gland survive if it is exposed to the juvenile hormone during the later stages of moulting? This was investigated by a modification of the technique previously employed (Wigglesworth, 1940) for switching over from adult development to larval development during the 5th stage. The 5th-stage larva was allowed to develop for 10 days or more at 250 C. The head was now left intact and a 4th-stage larva at 7 days after feeding joined to the abdomen.

The 5th-stage larvae were treated in this way at 10, 12, 14 and 17 days after feeding. The characters developed when they moulted were chiefly those of the adult (because development of most adult characters was already well advanced before the circulating juvenile hormone became effective), but the presence of juvenile hormone in these insects was proved by the development of partially larval characters in the abdominal cuticle in those operated upon at 10 and 12 days after feeding. In all of them the thoracic glands had completely disappeared by 3 days after moulting.

Clearly the thoracic gland cannot be protected from breakdown by exposure to juvenile hormone during the second half of the moulting period. It can be preserved only if ‘metamorphosis’ is prevented by the action of the juvenile hormone from an early stage.

Unless the thoracic gland has been exposed to the juvenile hormone from an early stage in the moulting period it breaks down within 24-48 hr. after the insect becomes adult; and since this change occurs in transplanted glands it must be determined humorally. That is, there must be some humoral change in the newly moulted adult which induces breakdown. Will this humoral change in the adult at the time of moulting induce the breakdown of thoracic glands of other insects exposed to it?

5th-stage larvae at 1 day after feeding were transfused with circulating blood from 5th-stage larvae 22 days after feeding—and continuously throughout the period of the next 3 or 4 days while the latter insects underwent their moult to the adult stage. The experiment was performed in two different ways: (a) by joining the head of the 5th-stage larvae at 22 days after feeding to the abdomen of those at 1 day after feeding; (b) by joining the head of the insect at 1 day to the abdomen of the insects at 22 days. In both series the younger 5th-stage larvae were left for 3-10 days after their partners had moulted, and were then dissected. In none had the thoracic gland been induced to break down.

The same experiments were performed using 4th-stage larvae 1 day after feeding for joining to the 5th-stage larvae during the final days of moulting. The thoracic glands of the young adults disappeared as usual within 2 days after moulting; those of the 4th-stage larvae were not affected.

In another series of experiments the thoracic glands from unfed 5th-stage larvae were implanted into newly moulted adults within 3 hr. after moulting. These were dissected 7 days later but in none had the implanted glands been induced to break down.

We must therefore conclude that two factors are concerned in the breakdown of the thoracic glands.

• (i) They must undergo some process of change (or ‘metamorphosis’) during the preceding moulting stage. Unless they have undergone this change they will not break down even when exposed to the environment in the newly moulted (or actually moulting) adult which induces the breakdown in its own thoracic glands.

• (ii) They must then be exposed to some further humoral stimulus which occurs at the time of moulting to the adult stage. After experiencing this further stimulus the glands apparently break down in any environment.

At what stage in the moulting process does this second change come about? A long series of experiments was carried out in which the thoracic glands were removed from 5th-stage larvae at different stages in the moulting process and implanted into the abdomen of recently fed adults. The adults employed were usually about 10 days old. The results may be summarized as follows.

• (i) Thoracic glands from 5th-stage larvae 7 days after feeding implanted; the adults dissected 13 days later. There were nine survivors. Thoracic gland cells had survived in all.

• (ii) Glands from 5th-stage larvae 14 days after feeding implanted; the adults dissected 12 days later. There were six survivors. In all of these the thoracic gland cells were conspicuous and actively secreting (that is, with enormous nuclei and abundant cytoplasm). In most experiments there were mitoses and other signs of moulting in the adult cuticle overlying the implant. In at least one case these changes were general over the surface of the abdomen.

• (iii) Glands from 5th-stage larvae 21 days after feeding implanted; the adults dissected 7 days later (the 5th-stage larvae of the same batch had all moulted by this time). Out of ten survivors, eight showed healthy nuclei surviving. This is well past the time when they would have broken down if they had been left in their normal position.

• (iv) Glands from 5th-stage larvae at 24 days after feeding implanted. (Most of the other larvae in this batch had already moulted; these would probably have moulted within 24 hr.) Among twenty survivors dissected 5-9 days after the transplantation, the survival of the thoracic gland was established in seventeen.

• (v) Glands were removed from adults at the time of moulting. In some the cuticle had just split and they were only part way out of the old skin, others had been out of the cuticle for a few minutes and others had moulted an hour or so previously. Among twenty surviving adults which received these implants three were dissected 4 days after implantation. In these some surviving nuclei of the thoracic gland could be seen, though many of them were in process of dissolution. Breakdown evidently proceeds more slowly in the newly transplanted gland. But in all the remainder (dissected 8-11 days after transplantation) the gland cells had degenerated—with the exception of one specimen (where the glands had been transplanted from an insect in the act of moulting) in which a number of apparently healthy thoracic gland nuclei remained intact.

Although it has not been possible by means of these experiments to fix precisely the moment at which the stimulus to breakdown is imparted to them, the results support the view that just around the time of moulting to the adult something happens to the thoracic glands which determines the subsequent breakdown of cells.

If the adult Rhodnius is decapitated immediately after moulting the dissolution of the thoracic gland is not affected: breakdown is complete in 48 hr. But we have seen that the stimulus which determines this breakdown is effective probably during the act of moulting or very shortly before. A series of 5th-stage larvae were therefore decapitated at 23 days after feeding. The time of moulting can be recognized by the appearance of air beneath the old cuticle. There were seven survivors: one was dissected at 1 day after moulting and showed the thoracic glands at an advanced stage of breakdown; six were dissected at 2 or 3 days after moulting and in these the thoracic glands had disappeared.

One must conclude from these experiments that the stimulus to breakdown does not come from the brain, suboesophageal ganglion, corpus allatum or corpus cardiacum. In Periplaneta the prothoracic gland requires some 12-14 days for its complete degeneration. But if the corpus allatum is removed, the brain and corpus cardiacum being left intact, the breakdown of the prothoracic gland can be prevented (Bodenstein, 1953). Bodenstein has therefore suggested that the presence of the corpora cardiaca in the absence of the corpora allata is responsible for the maintenance of the prothoracic gland of the adult.

Degeneration of the thoracic gland in the adult Rhodnius is determined so rapidly, and the anatomical relations are such that it has not been feasible to repeat Boden-stein’s experiment. As an alternative, the corpus cardiacum from the unfed 5th-stage larva, separated from the brain and the corpus allatum, was implanted into the abdomen of the 5th-stage larva 4-5 days after feeding. The resulting adult was then decapitated within a few minutes after moulting or, at latest, within 4-5 hr. In seven surviving adults the course of breakdown in the thoracic gland was not affected.

It is clear from these results that two factors are involved in the breakdown of the thoracic gland in the adult Rhodnius.

• (i) It must undergo some invisible change or ‘metamorphosis’ resulting from a moulting stage passed in the absence of the juvenile hormone.

• (ii) It must then experience some humoral stimulus at the time of moulting to the adult which induces in it an irreversible tendency to degenerate.

The nature of this second stimulus is unknown. But the phenomenon as a whole is exactly comparable with what happens in some other tissues of the insect at metamorphosis. For example, the larval fat body in Drosophila is conspicuous in the newly moulted adult but breaks down and disappears during the next 4 days (Wigglesworth, 1949). Indeed, at the moment of moulting, with or without metamorphosis, processes of many diverse kinds are set in motion, such as the darkening and hardening of the cuticle, the dissolution of unwanted muscles, and the discharge of the ‘cement’ from the dermal glands. These processes are presumably initiated by humoral stimuli but the source of the secretions concerned is unknown. Perhaps the neurosecretory cells which are so conspicuous in the thoracic and abdominal ganglia of Rhodnius and other insects may be concerned.

In general the results described agree with those reported by Bodenstein (1953) in Periplaneta. For Bodenstein also found that transplantation of larval corpora allata into young adult hosts does not prevent the breakdown of the prothoracic gland, and that larval prothoracic glands transplanted into old adult hosts fail to degenerate.

The most striking observation reported by Bodenstein was that removal of the corpus allatum from a newly moulted adult Periplaneta led to the persistence of the prothoracic gland and the moulting of the adult. No satisfactory explanation of this result has been found. But there is an observation on Rhodnius which may possibly have a bearing upon the problem.

If the thoracic glands removed from a 5th-stage larva 10 days after feeding (that is, at the height of their activity) are implanted into the abdomen of an adult female Rhodnius, this is induced to lay down a new cuticle—that is, to ‘moult’. But the process of egg development is not inhibited; most of the available food is diverted to the ovaries, and the new cuticle is exceedingly thin and delicate. On the other hand, if the corpus allatum is removed by decapitation at the same time as the thoracic glands are implanted, the deposition of yolk is inhibited and the moulting adult now lays down a new cuticle of normal thickness (Wigglesworth, 1952). It may be that in Bodenstein’s experiments, after the corpus allatum has been removed from a newly moulted adult the food materials are no longer diverted to the reproductive organs, the prothoracic gland resumes its activity and moulting occurs.

The thoracic gland in Rhodnius breaks down and disappears within 48 hr. after the moult to the adult stage.

Two factors are involved: (i) The gland suffers some change as the result of going through a moulting stage in the absence of the juvenile hormone. (ii) It must then be exposed to some humoral action at the time of the adult moult.

The source and nature of the stimulus which operates at the time of the final ecdysis are not known. But when the gland has been exposed to these two changes it will rapidly break down even in an environment containing the juvenile hormone.