AGRICULTURE AND BIOLOGY JOURNAL OF NORTH AMERICA ISSN Print: 2151-7517, ISSN Online: 2151-7525, doi:10.5251/abjna.2013.4.1.23.32

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Sensory receptors on abdominal and thorax segments in male and female red palm weevil Rhynchophorus ferrugineus

Mona Mohammed Saleh Al- Dawsary

College of Science and Humanities. Biology Department. Salman Bin Abdul Aziz University, Al- Kharj. Kingdom Saudi Arabia

ABSTRACT

The red palm weevil is a major pest infesting date trees in many countries located around the Middle East and Arabian Gulf region. To further describe the mechanism of insect- plant chemical communication, we observed the body segments and their sensilla of R.ferrugineus by scanning electron microscopy. In both sexes, there were twelve types of sensilla found on the thorax and abdominal segments: sex types of sensilla coeloconic, four types of sensilla trichoid, sensilla bifid and sensilla basiconica. We described morphological and discussed possible functions.

Keywords: Rhynchophorus ferrugineus, Thorax- Abdomen- Sensilla- SEM. INTRODUCTION

Chemical communications between insects and host plants is one aspect of insect chemoecology. By clarifying the mechanism of chemical communication between a moth and its host plant, chemoecological research provides the theoretical basis for integrated management. Receptors are the main tools of insect chemical communications and mainly located of the antennae (Quero et al., 2004; Liu, 2006). Receptors tune feeding preferences (De Boore, 2006), recognize host plant odors (Skiri et al., 2005), and play important roles in insect survival and environmental adaptation. The red palm weevil is the most dangerous pest of the date palm in the Middle- East region. An understanding of R.ferrugineus senses to environmental changes could be of considerable importance in formulating future control methods. In previous studies, the receptors of the red palm weevil’s head, mouth parts, legs, and external genetalia ( both males and females) have been identified (Sharaby and Al-Dosary,2006, 2007). The current research is a continuation to the previous study that was carried out on insect sense organs. The current study focused on studying the receptors to both thorax and abdomen for males and females; and to link the current results with previous studies with reference to the function of potential filaments which have been observed.

MATERIALS AND METHODS

Insects: Adults of R.ferrugineus were obtained from a physiology colony maintained by the insects laboratory at the college of science and humanities, Salman bin Abdul Aziz University, Al- Kharj, KSA.

Scanning microscopy studies (SEM): Freshly emerged adults females and males which were killed by chloroform solvent. Insect is dissected carefully in physiological saline solution to separate each of the sternum and Tergum. The tergum and sternum were first kept in 2.5% glutraldehyde for 24 h then dehydrated in a graded acetone series of 70%, 80%, 90% and 100% in each case for 1 h. The dehydration processes was followed by air drying. (Fine Serial No. PGO – 329 – Supply KPFREQ), then examination was made under scanning electron microscope (SEM) (JEOL – JSM 636 OLV). Tegum and sternum from five males and five females.

RESULTS

Description of Thorax and Abdomen: The thorax characterization in red palm weevil is divided into three parts: First, the prothorax cone length is equal to the length of the head andthe border side is non-clear, curled, and grows sensillae. The mesothorax is a small quadrate that is equal to the length of one-sixth the length a full thorax. The metathorax quadrate and pelura is clear and uncoated with elytra. The elytra with a soft velvety texture of which the summit of the end of the square does not cover the latest (upper or lower) segments of the abdomen. The hind wings are long and narrow (Viado and Bigornia, 1949). Second, the abdomen is flattened from the upper convex side of the ventral abdomen and narrows gradually towards the end of the posterior. Lastly, the tergum is semi-conical and the ventral side can be seen in eight shiny segments. The pigidium is semi-conical of which the, last edge is thick (Viado and Bigornia, 1949). The body wall curly looks like a sheet superimposed over each

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other (Figure1, 2). The cuticle of the abdominal thorax segments is densely coated with sensillae(Figure 3), In addition, multiporous on thorax segments were present which possible indicates

Thermo- Higroreceptors (Figure 4). Of additional note were a noted form and distribution similarity of sensillae on body segments in males and females.

Fig 1: Scanning electron micrograph of the red palm weevil showing surface of

cuticle thorax.

Fig 2: Scanning electron micrograph (SEM) of the red palm weevil showing surface area

of cuticle abdomen.

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Importance sensillae distribution on thorax and abdominal segments: Twelve morphologically different types of sensilla were identified and included: six subtypes of sensilla coeloconica; four subtypes of sensilla trichoidea, (differed in structure, size and density), one type of sensilla bifid (observed on the lateral and middle abdomen), and one type of sensilla basiconic ( present between other sensillae).

Coeloconic sensillae: Six number of coeloconic sensilla are distributed on the cuticular lateral and

ventral surface of all segments of the thorax and abdomen. It is a circular pit in the cuticle surrounded by many small pegs of trichoid sensillae at its circular edges (Figures 5: A; B; C; D, 6: A, B, and 7: A, B) . It is chemoreceptors as reported by (Schneider and Steinbrecht, 1968), and described by (Flower and Helson, 1971, 1974) coeloconic sensillae are observed in sex types:

Fig 3: SEM of the red palm weevil density of sensilla on the abdominal segment.

Fig 4: SEM of the red palm weevil pores present on the tergum abdomen (arrow).

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A B

Fig 5: SEM photomicrographs of sensilla coeloconica type I, II, III and IV.

A

D B

C

Coe I

Coe IV Coe III

Coe II

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Fig 6: SEM photomicrographs of sensilla coeloconica type V. A: Distribution of Sensilla Coe V. B:

Morphology of Coe V.

Fig 7: SEM photomicrographs of sensilla coeloconica type VI. A: Distribution of Sensilla Coe VI. B: Morphology of Coe VI.

A B

Coe VI

Coe V

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Fig 8: SEM showing distribution of Sensilla Bifid (SBF).

SBF

A

B

SBF

SBF

Fig 9 A : SEM showing distribution of Sensilla Bifid. B: Morphology of Sensilla Bifid (SBF).

SBF

SBF

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Coeloconic sensilla type I: In the circular pit in the cuticle, a long peg of the trichoid sensilla is projected from the center, while the circular rim of the pit is surrounded by a number of short trichoid sensillae (Figure 5A).

Coeloconic sensilla type II: In the circular pit in the cuticle, it is surrounded by numerous number of trichoid sensillae are radiated the circular rim of the pit (Figure 5B).

Coeloconic sensilla type III: Appears as coeloconic type II, but it contain three groups in the pit (Figure 5C).

Coeloconic sensilla type IV: Sensillae obsereved that line the edge of the cuticle surrounded by numerous number of trichoid sensillae (Figure 5D). Coeloconic sensilla type V: Sensillae coeloconic were distributed around the cuticular surface abdomen. A deep circular pit in the cuticle with a central peg protruded from the base pit. Additional numbers of trichoid sensillae surrounded the circular rim of the pit arising from an acute angle from the cuticle. These sensilla as homogeneously grouped in different length and thicknesses (Figure 6 A, B).

Coeloconic sensilla type VI: Observed as a pit in the cuticle with, a long peg of trichoid sensilla projecting from the center of the pit (Figure 7A, B). These sensillae were singularly distributed over the i l e e s abdominal segments only.

Bifid trichoid hair (Bf. Tr): Projected from the cuticle as bifid hair (Figure 8, 9: A, B), this type presents dense lateral and middle abdominal segments. Saïd et al. (2003) recorded the same sensilla on the antennal club of R.palmarum and mentioned that in TEM studies, the hair brunches arose in circular cuticle sections, either in contact or distant, according to the level of the cut.

Trichoid sensillae (Figures 10- 12): Four types of trichoid sensillae were observed and included:

Trichoid type I (Tr I): This sensilla is characterized as long, thin wall, aporous, with a sharp edge was very abundant on all the abdominal segments ( Figure 10).

Trichoid type II (Tr II): A sensilla from a pit on the cuticle, the sickle- shaped and, multi- porous, with an immediate straight body followed by bend from the op n le of 45˚ m e p on he i le n of, variating lengths ( Figure11: A, B).

Fig 10: SEM photomicrographs showing distribution of sensilla trichoidea TrI

Trichoid type III (Tr III): This hair multi- porous, its wall is curly, long to take the curve as moved to the top ( Figure 12: A).

Trichoid type IV (Tr IV): Longitudinal, thick wall, multi- porous, and of different lengths ( Figure 12: B).

Trichoid sensilla were identified on the thorax and abdominal segments of males and females of R.ferrugineus. The sensilla trichoidea type I observed in this study showed external characteristics of a porous sensilla.

Basiconic sensilla (BS): Short basiconic sensilla were identified on the abdominal segments of both red palm weevil sexes with multi- porous sensilla and pores throughout their walls. The length of these sensilla were approximately 20 µm (Figure 13). Generally, these sensilla do not curve and were found in large numbers between other sensilla. Sensilla basiconica are structurally and functionally similar in most insects species (Schnider, 1964). In this study SB had pores from the base to the tip. The larger numbers of SB on the abdominal segments of males and females may be related to finding food or suitable habitats.

TrI

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Fig 11 A: SEM showing various different distribution types of Trichodea sensillae. B: Morphology of Sensilla

trichoidea type II.

Fig 12: SEM showing morphology of Sensilla trichoidea type III, IV. A and B.

A

B

B A

ST II

ST IV

ST III

20 µm

20 µm

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DISCUSSION

Twelve different types of sensilla were identified on the thorax and abdomen of males and females red palm weevil R.ferrugineus. All sensillae types presented, in this study with displayed external morphologies similar to the previous study witch described with Sharaby and Al-Dosary (2006, 2007). The noted study indicated sensillae on head, mouth parts and external genetalia, but with different abundance and location. In addition, males was sensillae numerous compared females according to (Sharaby and Al-Dosary, 2006, 2007). The same morphological types of sensilla were observed in both sexes. The coeloconic sensilla observed in this study showed circular pit in the cuticle surrounded by many small pegs of trichoid sensillae. Coeloconic have been reported as chemoreceptors (Schneider and Steinbrecht, 1968), and described by (Flower and Helson, 1971, 1974). In social insects, such as ants and wasps, whose behavior is guided by variations in temperature, soeloconic are chemoreceptors that respond to air temperature changes (Ruchty et al., 2009). In this study soeloconic sensilla of different types found differations numbers surrounded by ST, and any of the aforementioned can be presumed. In some lepidopterans these sensilla are reported to have receptor neurons that respond to host plants´ volatile compounds (Pophf, 1997). Some authors

even considered that this type of sensilla may participate in receptivity to heat, humidity, and CO2 (Alterner et al., 1983; Cuperus `983). Bifid trichoid (Bf. Tr) observed on middle abdominal segments (Fig 8, 9 A and B). The observation revealed neither wall pores no dendrites. A narrow central channel surrounded by electron dense areas was noticed, and was, disposed in circule at the periphery of the hair. This is assembly protective function (Saïd et al.2003) and did not seam to have sensory function similar to the non- innervated hair– like structure described in Conotrachelus nenuphar (H.) (Alm and Hall, 1986) and Hylobius obietis (L.) (Mustaparta, 1975). The trichoid sensillae presented show on segmental body and four types were long, multi- porous (Tr type II, III, IV) except Tr I of which were a porous, and probably function olfaction. Trichoids hairs have been reported to perform either or both mechano- and chemo- sensory functions as described by Zacharuk (1985). ST I additionally described the antenna of other heminopteran families ( Dickens et al., 1995; Chinta et al., 1997). The sensilla trichoid ST II, ST III, ST IV were multi porous and thick of wall , that Tr II Tr III curved at the tip. Bland (1981) mentioned that multiporous sensillae are a major receptors of odor and or water vapor. In a previous study we found similar trichoid sensillae in the current study on the same insect on each of the

Fig 13: SEM showing morphology of Sensilla Basiconic

SB

20 µm

20 µm

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antenna ovipositor, copulatory organ (Sharaby and Al- Dosary, 2006, 2007). Sensilla basiconica are structurally and functionally similar in most insect species studied (Schnider, 1964). In this study BS short, identified the abdominal segments of both sex and multi- porous. Some authors have indicated a thermo- or chemoreception function to these sensilla ( Chapman, 1982, Zacharuk, 1985).

This study built upon previous R.ferrugineus sense organs studies and the results presented here are important for ongoing research efforts on behavior in these species.

ACKNOWLEDGMENTS

This work is part of a research project supported by the Deanship of Scientific Research No. 37 / H / 1432 H.

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SBF