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CAPTIVE BREEDING MANUAL FOR BEETLES OF THE FAMILY SCARABAEIDAE,  SUBFAMILIES CETONIINAE (FLOWER BEETLES) AND DYNASTINAE (RHINOCEROS BEETLES)

BY:  C. CAMPBELL

(page 6)

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Pupal cell of Chalcosoma caucasus - Image  C. Campbell
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Fig. 37:  The puation cell of C. caucasus.  This cell measures about 17 cm (6.75 in.).  Even large cells such as this can prove delicate, and care should be taken when working around them.
(i - pupation)

Many cetoniine scarabs complete their larval phase in just a few months, whereas some of the really large dynastines take a year or more.  When your scarab larvae reach the maximum growth point in the late 3rd instar phase, they will slow down in their eating, and begin construction of their pupal cells.  The pupal cell of a scarab larva is basically the same as the cocoon of a moth.  However, whereas a moth's cocoon would be made of silk produced by the caterpillar, the pupal cell of a scarab is instead made of bits of wood and leaves cemented together.  The cement used to stick all of the wood/leaf particles together is produced by a special gland in the larva.


 
The function of the pupal cell is to seal off the larva and protect it from the outside world while it undergoes the delicate process of transforming to the adult stage.  Scientifically, the adult beetle is called an "imago".  This same term is used to describe the adult stage of all insects.  When the time for your larvae to pupate is nearing, you must exercise additional caution whenever digging around in the substrate to perform periodic substrate renewals, or just routine checks on the condition of the larvae.  The extra careful treatment is necessary because if you were to accidentally damage the pupal cell of a larva as it was constructing it, it could upset the larva greatly and threaten its very survival.  Some species of scarab larvae are not readily able to repair their cells once they have been accidentally damaged.  The pupal cells of cetoniines are usually created in the upper and middle layers of the substrate.  Dynastine larvae nearly always build their cells at the lowest possible level, often using the floor of the rearing container as the bottom wall of their cell.  Cetoniine larvae often construct their cells against the wall of the terrarium as well, but they use the side walls rather than the floor.

 
If you are using a very transparent container such as a glass aquarium, glass bottle, or clear plastic box, this habit of building the pupal cell against the walls can often allow you to have a window into the cell throughout the entire metamorphic process.  In most cases however, the majority of your larvae will build their cells away from the walls of their rearing terrarium, and will rest freely within the substrate.  This is especially true of the cetoniine species.  In dynastines however, the cell is almost unfailingly built tightly attached to the very bottom of the terrarium.  Cells such as those are best left undisturbed, but the cells of smaller cetonniinae which are not attached to walls or other objects can be handled or moved prior to emergence of the adult beetle if needed.  Be very careful if you handle them, because the structure of some cells is quite delicate, and can be ruptured if handled carelessly.  Handle them carefully as if they were fragile eggs.  Even the large cell of a dynastine scarab can appear deceptively robust and strong, but really they are not and can be easily damaged through rough handling.  If you come across cells while doing a routine substrate change, and they need to be moved, pick them up very carefully, as they don't become very solid until the larva has completed work on the inside.  Virtually no pressure should be applied to the cell when picking it up at any time.  The cell should be placed back in the substrate at a similar level to that in which it was found.  If you ever happen to break a cell, it is best to just place it back in the soil, fitted together as best as possible, and cover it over.   If the larva inside hadn't finished cementing, it will usually repair the damage.  If the cell already contained a pupa, the break will not be repaired and the pupa may mold and disintegrate.
Pupa of Chalcosoma - Image  Al O Myrrhina
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Fig. 38:  A large male pupa of Chalcosoma, here removed from its protective cell for photographic purposes.  If done extremely carefully, it can be placed back inside its cell undisturbed, but ordinarily, pupae should not be removed from their cocoons for observation by anyone except the most experienced of beetle breeding hobbyists.  In such cases, the cell is carefully and precisely separated into two separate halves which can  neatly be placed back together, leaving no opening to the outside environment.  Ordinarily, the only reason to do this would be in instances where one would seriously want to document the various life stages in their entirety.  Without a surrounding cell of precisely the right shape, proper transformation to the adult stage is impossible, and would cause great physical deformity upon emergence from the pupal skin.  The reason is because the pupa must undergo a series of specific bodily rotations when emerging from the pupal skin, and it uses the cell walls around it for leverage.  If not able to rotate in a natural manner, failure to emerge properly is almost inevitable.
Photo courtesy of Al O Myrrhina.

 
Once all of the larvae within a rearing container have made their pupal cells, no further substrate changes or food additions will be necessary.  Pupation time varies greatly depending upon species.  Many of the small to medium cetoniines can do it in 4-6 weeks, but most rhinoceros beetles take at least a few months.  In many cases, the larger the species you are breeding, the longer you can expect larval durations to be, and the longer you'll have to wait for pupation to be completed.  Your beetles will not all be emerging from their pupal cells at the same time, just as not all of them pupated at the same time either.  Instead, you will have a period of time in which they will be emerging gradually, until eventually, all have come out from their underground cells.  At this time, you will basically be back at the same point where you first started (assuming that you began with adult beetles rather than larvae), and you will be ready to breed your beetles all over again. 

A question that often comes up in the hobby of captive rearing beetles is whether or not repeatedly breeding the same individuals back together is genetically harmful to them.  The answer is "possibly", but in my personal experience, I have never observed any obvious abnormalities caused by inbreeding.  When beetles occasionally turned up that had physical deformities, it was always readily attributable to an error in the pupation process, rather than a genetic abnormality.  If it is possible for you to introduce new genetic stock to your culture of a beetle species every few generations, then that is something which you may wish to consider doing.  However, in my personal captive breeding efforts with scarab beetles and many other insects, I have not found any problems linkable to inbreeding, and it would appear that insects are not affected by this process to the extent that vertebrate life forms are.

Lastly, I would like to point out that the information presented in this manual is largely the result of my own personal work experience with about a dozen different species of cetoniine and dynastine scarab species.  There are a number of other pages on the internet that you should definitely visit to get a more rounded idea of the techniques used by other hobbyists.  I have included some important links below.  Some contain general instructions for the breeding of all cetoniine / dynastine scarabs, while others are concerned with the specifics of rearing certain species.

World of the Beetles  - (general)
Breeding Megasoma acteon - (Megasoma actaeon)

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