ORIGINAL ARTICLE

Integrating laboratory and field studies on the biology of Oryctes

rhinoceros (Coleoptera: Scarabaeidae)

O. Veena,1 C.V. Lekshmipriya,1 T.S. Swapna,1 S. Sreekumar2

1Department of Biotechnology, University of Kerala, Kariavattom Campus, Thiruvananthapuram,

2Department of Zoology, University College, Thiruvananthapuram, Kerala, India

a focus on its life cycle, developmental stages, and ecological interactions. Under both laboratory and

field conditions, the insect's life cycle was observed to include three larval instars, followed by a pupal

stage, before emerging as an adult. The third (final) instar larvae have an extended duration of up to

120 days and are voracious feeders, consuming approximately 4.5 grams of cow dung per day. Prior to

pupation, older larvae enter a non-feeding stage known as the prepupae. During this stage, they

construct cocoons from cow dung, within which pupation occurs. Newly emerged adults remain inside

the cocoon for 20–25 days, during which time their reproductive and digestive systems mature. Larvae

and adults exhibit different feeding habits; larvae are detritivores and adults phytophagous. Field

surveys have identified cow dung pits and decaying organic matter as the primary breeding sites. Under

mass-rearing conditions, synchronized pupation was observed, potentially mediated by larval

The life cycle of insects generally begins with embryonic development within the egg, triggered by

fertilization. The egg provides both shelter and nourishment for the developing embryo. By the time of

hatching, the embryo will have developed into a tiny larva crammed inside the eggshell.[1] The post-

embryonic development of an insect is commonly referred to as metamorphosis. This developmental

process, in which the first instar larva transforms into an adult, is called metamorphosis, meaning

“change in form”.[2]

Insects can be divided into two groups based on the type of metamorphosis they undergo: Apterygota

and Pterygota. Apterygota are primitively wingless insects that do not undergo any significant change

in form. In Apterygota, metamorphosis is either absent or only slightly indicated. The immature instars

differ from the adults only in size and the absence of genitalia. These insects are also known as

referred to as exopterygotes. Holometabolous insects exhibit immature stages that differ significantly

from the adult in terms of morphology, physiology, and feeding habits. The morphological gap between

the immature stages and the adult is bridged by a distinct, semi-quiescent stage known as the pupa.[3]

Holometabolous insects undergo complete metamorphosis and are also called endopterygotes, referring

to the internal development of wings.

‘Eclosion’ is a general term used to denote the emergence of an adult insect from the pupal case, or a

larva from the egg. The emergence of the larva from the egg marks the beginning of the first stadium

in the development of a holometabolous insect, and thereafter the immature stages are known as larvae.

In Coleoptera, the incubation period varies from a couple of days to months. The exit of larvae from

the eggshell is brought about by tearing the shell through its line of weakness. In some Coleoptera and

stadium. [7,8] The form of an insect between successive moults is referred to as an instar, the time interval

between two successive moults is known as the stadium, and the level of larval development is denoted

by the term stage. Hinton (1958) and Snodgrass (1935) have suggested that a new instar typically

begins with apolysis, the process involving the separation of the old cuticle before the secretion of a

new one. [9,10]

In the development of holometabolous insects, a great many variations exist in the morphology of the

larvae.[11] Based on morphological adaptations, the endopterygote larvae are grouped into oligopodous,

polypodous and apodous forms. The oligopodous larvae lack abdominal prolegs but have functional

thoracic legs and prognathous mouth parts e.g., Neuroptera and Coleoptera. Polypodpus larvae are

cylindrical with short thoracic legs and abdominal legs e.g., Lepidoptera and Hymenoptera. Apodous

mostly in the pupal stage. The "pharate adult" stage in insects is the period when the adult insect is fully

formed but remains inside the pupal exoskeleton and hasn't yet emerged. This stage occurs after the

pupal-adult apolysis. Typically, a protective cell or cocoon surrounds the pupa and the immature pharate

adult. Certain Coleoptera, Diptera, Lepidoptera and Hymenoptera have unprotected pupae. Hinton

(1964) classified pupae according to the presence or absence of articulated mandibles used for escaping

from a cocoon.[13] The pupae having such mandibles are described as decticous, e.g., Neuroptera,

Mecoptera, Trichoptera and certain Lepidoptera families. Pupae without functional mandibles are

adecticous, e.g., Strepsiptera, Coleoptera, Hymenoptera, Diptera and Siphonaptera. Based on whether

the appendages are free or remain adherent to the body, the pupae are classified into exarate and obtect

types. Exarate pupae have appendages free and are not covered by cocoon; but in obtect pupae, the

Beetles constitute the largest and possibly the most economically important family of insects in the

world. Beetles comprise approximately 40% of all species of insects and 25% of all animal species.

Beetles are found in a wide variety of habitats. Many beetle species are herbivorous or predatory, while

others are scavengers or fungivores.[14] In some cases, the different life stages of beetles may exhibit

different feeding habits; for example, the larvae of Oryctes are detritivores, whereas the adults are

phytophagous. Oryctes rhinoceros is a serious pest of the coconut palm. It is a holometabolous insect

with a lifecycle comprising a detritivorous larval and phytophagous adult phases. As in all

holometabolous insects, an intermediary resting or pupal stage is included in the metamorphosis of

Oryctes rhinoceros. The biology of Oryctes rhinoceros was reported earlier by Grissett (1953),[15]

Kurian and Pillai (1964),[16] Catley (1969),[17], Bedford ( 1976,1980, 1983),[18-20] Sreekumar (1991),[21]

Material

Larvae and eggs of Oryctes rhinoceros, collected from the field, were used for the study.

Rearing

The eggs and larvae, collected from the field, were reared in the laboratory following the method of

Sreekumar and Prabhu (1988) with some modifications.[24] Small plastic containers of 9 cm height and

7 cm diameter were used to rear the larvae. One-third of the bottle was filled with cow dung, which

served as food for the larvae. To make the medium free of pathogens, cow dung was steam sterilised

and cooled before use.[19] To facilitate the passage of air, holes were drilled in the lid of the container.

The medium inside the container was changed on alternate days. The larvae were maintained singly in

Method of mass rearing

Mass rearing of the larvae was done in a glass trough of diameter 1.5 m, two third of which was filled

with the sterilized cow dung. The larvae collected from a single cow dung pit were transferred to the

troughs after brushing each with the camel brush for removing the mites. The larvae were predominantly

third instar and varied in weight and size. The mass culture was left undisturbed for two weeks.

Field Studies

Observations related to the biology of Oryctes rhinoceros were also recorded during field visits.

Insect development under Laboratory conditions

The lifecycle of the Oryctes rhinoceros included three stages; larva, pupa and adult. Observations on

the various stages of development of Oryctes rhinoceros under laboratory conditions are mentioned

developing larva protruding from the shell. The larva then emerged from the egg with wriggling

movements. Shortly after emergence, the larvae remained quiescent until they were ready to feed. The

remnants of the eggs served as the first food for the newly emerged larvae.

First instar larvae

The newly emerged larva had a much wrinkled and transparent body measuring 1.55 ±0.29 cm in length.

In the newly emerged larvae, the head capsule and mouth parts, except the tips of mandibles, were soft

and creamy white; later, these structures became harder and darker. The larval body gradually became

broader and more elongated, followed by the straightening of the thoracic legs. The brownish black

head capsule measured 0.28±0.06 cm in width (Table 1). The initial weight of the first instar larvae was

0.02±0.05 g. Later, they attained a weight of 1.41-1.53 g before moulting to the second instar. The

medium. The legs were hairy and well-developed, with backwardly directed chitinous spines. The

female beetle also could be distinguished from the male by the presence of a densely haired pygidium.

A pair of hard sclerotised elytra was visible on the dorsal side of the body. The hindwings were hidden

inside the elytra. The elytra protect the wings against injuries and help the concurrent closing of the

common breeding sites for rhinoceros beetles, a finding consistent with the present study. [16,25] In

Burma, dead coconut stems, rotting paddy straw heaps, and farmyard manure have been reported as the

primary breeding sources for the beetle.[26] In the present study, Oryctes larvae have also been recovered

from decaying stems of Murraya oleifera and papaya plants. This may be the first report documenting

decaying papaya stems as a breeding site for Oryctes rhinoceros. It is observed in the present study that

As revealed by the present study, the life cycle of Oryctes rhinoceros includes three larval instars,

followed by pupal and adult stages. The adults feed on the tender parts of coconut palms, whereas the

from 0.10 to 1.8 g. [15,17,19] The larvae are initially transparent with the head capsule and mouth parts

appearing soft and delicate. These structures later become chitinized and hardened. As with most

Coleopterans, the larvae remain motionless briefly after moulting, then resume activity as feeding

begins. The stages and durations of larval instars observed in this study generally agree with the reports

by Bedford (1980) [19] and Sreekumar (1991).[21] Any observed variation in larval stage duration may be

influenced by the quality of the food.

Field surveys have revealed that larval colonies are often dominated by third instar larvae. The larval

colony consisting entirely of any particular stage is very rare, which is consistent with the observation

of Bedford (1976).[18] Fully grown larvae, weighing 12–14 g, cease feeding and gradually lose weight

prior to transforming into prepupae. Sreekumar (1991) reports that as the Oryctes larvae become older,

they enter a period of endogenously induced state of starvation prior to their transformation into

prepupae.[21] The prepupa purges gut contents, especially from the proctodaeal dilation, before pupation.

Various studies indicate that successful pupation requires larvae to either attain a critical weight [28] or

In the present study, the prepupal period is found to last for 8-10 days, in agreement with earlier reports.

outer casing, the cocoon. The larvae can make use of a variety of materials for cocoon construction.[30]

For example, in more advanced groups of flies (Diptera), the skin of the last instar is hardened into a

adding extra materials from the surroundings such as bits of leaf, particles of sand or even faecal pellets.

The majority of coleopterans, however, construct cocoons using extraneous materials such as soil or the

food medium itself; e.g., Rhynchophorus ferrugineus constructs cocoon from the chewed fibrous

materials of the host plant.[30] Older larvae of Oryctes rhinoceros construct cocoons using the breeding

medium itself, as reported by Sreekumar (1991).[23] Within the cocoon, larvae transition through

prepupal, pupal, and adult stages. Both the prepupal and pupal stages are quiescent with visible external

segmentation. When disturbed, prepupae wriggle, while pupae exhibit up-and-down movements.

Sexing is possible at the pupal stage as male pupae can be distinguished from females by a relatively

longer, upward-directed horn on the head. The rarity of pupae at breeding sites observed in this study

can be attributed to the short duration of the pupal stage and their concealed nature within cocoons made

after adult emergence. After the reproductively active period, females cease feeding, become inactive,

in Figure 4, a plot of the logarithm of head capsule width against instar number yields a straight line,

consistent with Dyar’s rule. Similarly, the near-uniformity of RW values across instars further supports

the accuracy of both instar count and identification. All the earlier reports suggest a three-instar pattern

under laboratory conditions. Synchronized pupal colonies contain pupae of variable sizes. This

observation indicates that third instar larvae at varying stages of growth underwent pupation in response

to some external cues. Environmental factors such as light, temperature, and other exogenous or

endogenous cues may influence pupation. The older larvae of Oryctes rhinoceros discharge mandibular

secretions and gut contents to the medium during the construction of cocoon. It is suggested that the

This study elucidates the biology of Oryctes rhinoceros, a destructive holometabolous pest of coconut

palms, by integrating laboratory and field observations. The insect’s life cycle includes three larval

instars followed by a pupal stage within a cocoon composed of cow dung. Pupation is observed to be

triggered upon reaching a critical larval weight (12–14 g), while discrepancies in developmental

timelines highlight the influence of environmental factors including the quality and availability of food.

The newly emerged adults remain inside the cocoons for an additional 20–25 days, during which the

revealed synchronized pupation, wherein larvae kept in groups pupated simultaneously, forming

clustered cocoons. This phenomenon, not previously reported in Oryctes rhinoceros, is hypothesized to

be mediated by semiochemical cues from larval mandibular secretions discharged during cocoon

Conflicts of interest

There are no conflicts of interest.

References

3. Hilton W A. The morphology and development of insect metamorphosis. Washington, DC: Smithsonian

24. Sreekumar S, Prabhu VKK. Digestive enzyme secretion during metamorphosis in Oryctes rhinoceros (Coleoptera;

precocious metamorphosis in Oryctes rhinoceros (Coleoptera; Scarabaeidae), Proc Indian Acad Sci (Anim. Sci.)

30. Kumari AYK, Sreekumar S. Cocoon construction by larvae of Rhynchophorus ferrugineus (Coleoptera:

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