Original Article

Morphology and distribution of maxillary sensilla in the larvae of

Oryctes rhinoceros (Coleoptera: Scarabaeidae)

R. Neelima2, M. Anandhu1, V.M. Adhikesh3, Ramya R. Prabhu2, T.S. Swapna1, O. Veena1

3 School of Electrical Engineering (SELECT), VIT Chennai, Tamilnadu, India

feeding and breeding. This report describes the mouthparts of Oryctes rhinoceros larvae and the

different types of sensilla present on the maxillae. The larvae possess mouthparts specialised for

biting and chewing, commonly referred to as the mandibulate type. These comprise five principal

parts arranged from dorsal to ventral side viz., the upper lip or labrum, a pair of sclerotised and

toothed mandibles, a pair of maxillae, the lower lip or labium, and the hypopharynx; all of which

operate together to cut, grind, and manipulate solid food. Insects rely on their sensory systems to

detect environmental cues and assess food quality, both of which are critical for feeding, growth,

and reproduction. Feeding is regulated by sensory structures located on mouth parts, known as

sensilla, which detect phagostimulants, deterrents, and the physical characteristics of the food.

Among the mouth parts, the maxillae serve as the most important sensory structures and were,

sensilla chaetica, trichoid sensilla.

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Introduction

Insects rely on their sensory systems to detect environmental cues and monitor the quality of food

they eat. These systems are critical for behaviours like food selection, which is vital for the growth

and development of immature stages (larvae), metamorphosis, and reproductive development in

adults.[1] Oryctes rhinoceros larvae are voracious, nonspecific feeders of detritus.[2] The regulation

of feeding in insects involves a complex interplay of neural and endocrine signals in response to

both internal and external stimuli.[1,3] Internal stimuli are mediated by singly distributed gut

endocrine cells, neurosecretory cells located in the brain and ganglia of the stomatogastric nervous

sensilla, which are integumentary processes rich in nerve endings. Most of the sensilla act as

chemoreceptors, detecting chemical cues such as phagostimulants (substances that stimulate

feeding) and deterrents (substances that discourage feeding).[4] The mouthparts are also equipped

with various types of mechanosensory sensilla that may detect the physical characteristics of the

environment and food. The number and structure of sensilla on mouthparts and antennae may vary

depending on the complexity of the insect’s chemical environment, such as food availability or

host plant chemicals. This adaptability is known as plasticity.[1] This report describes the

morphology of the maxilla and various sensilla associated with it in the final instar larvae of

Oryctes rhinoceros.

Materials and Methods

surface sterilized by wiping with a cotton dipped in 70% ethanol before dissection to remove debris

and potential ectoparasites from the surface. Next, the etherized larvae were firmly secured, ventral

side up, on a wax-bottomed Petri dish, placed under a dissection microscope. The mouthparts were

removed sequentially using a pair of fine-tipped forceps and microscissors. This proceeded in the

anatomical order in which they were arranged: isolating first the labium, then the maxillae

followed by the mandibles and hypopharynx, and finally the labrum. Each isolated mouthpart was

dipped in insect saline to prevent desiccation and maintain tissue turgor. The cleaned mouthparts

were carefully arranged on a glass microscopic slide in their correct anatomical sequence. The

specimen was then documented photographically for detailed morphological examination.

Scanning electron microscopic examination of the maxilla

Hitachi) for 30 min. They were mounted on a SEM specimen stub with double-sided adhesive tape

and sputter-coated with gold palladium using an E101 ion sputtering unit (Hitachi). The processed

specimens were viewed under the SEM at magnifications of 40X and 1000X, and images of

maxillary sensilla were taken. Counting and measurements of sensilla were done using the

software ‘Image J’.

Observations and Results

The mouthparts of Oryctes rhinoceros larvae consist of the following structures: labrum, a pair of

mandibles, a pair of maxillae, and labium (Figure 1).

The labrum or upper lip, is a single flap located on the dorsal side. Just below it lies a pair of

mandibles, which are heavily sclerotized (hardened) and toothed. These function as jaws for

grasping, cutting, and crushing food. Situated beneath the mandibles is a pair of maxillae, which

assist in manipulating food. On the ventral side is the labium, or lower lip, which is a median flap.

The labium plays a role in both food manipulation and sensory perception. The hypopharynx is a

tongue-like structure situated between the labrum and the labium.

Structure of the maxilla

partially fused and can be identified separately only from the underside. Of the two structures, the

galea can be identified as the outer lobe located between the maxillary palp and lacinia. Lacinia is

the larger lobe placed inside and has stridulatory teeth that are truncated (Figure 3). The maxillary

palps are five segmented appendage-like structures held by stipes. The palps are sensory structures

that help in food selection.

Figure 2B: Dorsal view of the Maxilla

Sensory structures of the maxilla

covered with different types of sensilla. The sensilla were observed to be primarily of the trichoid

and basiconic types (Figure 3).

Under SEM, the trichoid sensilla were categorized into two main types. The first type consisted of

blunt-tipped sensilla, which were thick and long with a rounded apical end. The second type

comprised pointed-tipped sensilla, which were thin and elongated with tapering tips (Figures 3,

4A and 4B). The trichoid sensilla could also be categorised into different types based on length

(Table 1). They remain anchored within the sockets formed from the integument.

Basiconic Sensilla (cylindrical), F= Coeloconic Sensilla (cylindrical), G=Coeloconic Sensilla (saucer-shaped),

The pointed-tipped sensilla were located mostly on the basal part of the lacinia, with a few

occurring on the basal region of the galea and on the stipes. They exhibited an attachment angle of

0.83 ± 0.04° and a length of 4.53 ± 1.82 µm.

Table 1: Structure and distribution of Trichoid Sensilla and Sensilla Chaetica in Oryctes rhinoceros

*Each number represents Mean ± Standard Deviation of 10 observations

These were long, hair-like bristles with a tapering tip, like trichoid sensilla (Figures 3, 4C and 5).

However, they were sturdy and had a thick-walled cuticle, typically set in a socket, occasionally

found very deep. The angle of attachment was 23.30±6.22o and measured 0.27±0.02 µm in length.

They were found to be densely distributed on the galea, particularly towards the apex and the basal

region. They were also present on the apical parts of the lacinia and maxillary palps. Although

they occur on both the upper and lower surfaces, most of them are oriented towards the interior,

i.e., the food-facing side.

These structures possessed a prominent anchoring region and resembled a peg. Some were pointed,

with a broad base and a tapering tip, while others were short and cylindrical. Both, pointed and

cylindrical types were abundantly present on the galea, often occurring in clusters (Figures 4D, 4E

and 5).

galea (Figures 3 and 4H).

The maxilla, being the most important sensory structure among the mouthparts, possesses various

types of sensilla. The most prominent sensilla present on the maxilla are the trichoid types. They

are slender structures remaining anchored within the sockets of integument, permitting free

movement.[4] Among the different trichoid sensilla found on the maxilla, the pointed types are

supposed to have olfactory and chemo-mechanosensory function, whereas the blunt-tipped types

are found to play only a chemosensory role.[9] It was observed in this study that in Oryctes

rhinoceros larvae, the blunt-tipped trichoid sensilla were predominantly distributed on the apical

some appear like pegs, and a few of them are saucer shaped. They are less numerous and are

located mostly on the galeae and lower surface of the apical parts of the palps. In Hylamorpha

elegans, sensilla coeloconica are significantly more numerous in males than in females.[25] In

Bombyx mori, the neurons of the coeloconic sensilla respond to a range of green leaf volatiles (C₃–

C₁₀), as well as to certain aromatic compounds and monoterpenes.[26]

Böhm's bristles are tiny, specialized hair-like mechanosensory structures found on the antennae

and function as proprioceptors.[29] The present study shows that in Oryctes rhinoceros larvae, these

are located at the outer rim of the stipes of the maxilla. They are observed to be present at the base

between the scape and the pedicel of antennae in Eucryptorrhynchus chinensis and

Eucryptorrhynchus brandti.[29] In Paussus favieri they are observed as thorn-like bristles, sharp

or blunt-tipped, straight or slightly curved, set in a wide socket.[15] In Phyllotreta striolata, Böhm

bristles are surrounded by a deep cuticular socket, and the slender conical shaft is smooth and

probably movable.[30] The concentration of Böhm sensilla at the intersegmental joints, between the

scape and the head, as well as between the scape and the pedicel, in Bembidion properans suggests

that these sensilla probably perceive the antennal position and movements. It is proposed that these

(2000) have noticed campaniform sensilla at the base of the antenna.[32] In Paussus favieri the

campaniform sensilla are represented by cuticular caps, emerging from large sockets. The cap is

cylindrical and sub-truncate at the apex, with a small pore in the middle. Cross sections show a

peg-in-pit appearance occupying a round cavity. The cuticular walls of the sensillum are thick and

without pores in Paussus favieri.[33] These sensilla in carabid beetle larvae are regularly distributed

on the labial palp surface, and additionally, they form a ring around the apex. The external body

is dome-shaped, and a flexible socket gives the cuticular insertion. On the maxillary palps of

Calathus mollis, their distribution and size are similar; however, two rings are usually evident.[14]

Placoid sensilla or pore plates are chemoreceptors, primarily involved in olfaction. [34] The present

study shows that in Oryctes rhinoceros, these projecting plate-like sensilla with multiple pores are

in females, suggesting their role in the detection of female pheromones during mating.[37] They

consist of thin oval cuticular plates, with no difference in size between sexes. In Hylamorpha

elegans, the plates of the placoid sensilla containing pores are seen bulging outwards.[25]

Our study reveals that the maxilla of Oryctes rhinoceros bears a complex array of morphologically

and functionally distinct sensilla, including trichoid sensilla, sensilla chaetica, basiconic sensilla,

coeloconic sensilla, placoid sensilla, and Böhm sensilla, in addition to a small number of singly

distributed cuticular pores and hairs. All these structures are associated with feeding and are

specialized for detecting various physical and chemical stimuli. Among the mouthparts, the

maxillae are regarded as the most important sensory structures; nevertheless, they are not the only

ones that bear sensilla. A comprehensive investigation of feeding-related sensilla should therefore

Conclusion

for consuming solid food. The maxillae play a crucial role in feeding, facilitating food selection

and rejection, processes that are aided by the diverse sensilla distributed on them. This adaptation

is particularly important since the larvae are non-selective feeders and rely entirely on the feeding

medium itself for their development and metamorphosis.

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