Distribution of olfactory and some other antennal sensilla in the male click beetle Agriotes...

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PII: SOO20-7322(97)00013-S

Inr. J. Insrcr Morph/d & Embrwl., Vol. 26, No. 2, pp 75-83, 1997 (:m 1997 Elsevm Scm~e Ltd

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DISTRIBUTION OF OLFACTORY AND SOME OTHER ANTENNAL SENSILLA IN THE MALE CLICK BEETLE AGRIOTES OBSCURUS L. (COLEOPTERA : ELATERIDAE)

Enno Merivee*$, Mart Rahit and Anne Luik*

*Institute of Plant Protection, Estonian Agricultural University, 64 Kreutzwaldi Street, EE2400 Tartu, Estonia tInstitute of Zoology and Botany, 18 1 Riia Street, EE2400 Tartu, Estonia

(Received 28 January 1997; accepted 23 May 1997)

Abstract-The distribution of 5 types of sensilla was statistically analysed on the 410th antenna1 segments of the male click beetle Agriotes obscurus (Coleoptera : Elateridae). The distribution pattern of the trichoid pheromone receptors (T, sensilla) and the olfactory basiconic B,B, sensilla on the antennae of male A. obscurus differs significantly from the distribution pattern of the contact chemoreceptors (T, sensilla) and probably the non-olfactory B, and D sensilla. A significant peculiarity of the distribution of olfactory sensilla is their location on the antenna1 segments as 2 separate (dorsal and ventral) fields of sensilla. The numbers of T, and B,B2 sensilla on dorsal fields of sensilla of the 410th segments increase towards the apex of the antenna nearly linearly. On ventral fields of sensilla of the Cloth antenna1 segments, the number of B,B, sensilla is nearly uniform; the number of T2 sensilla in the proximal part of the antenna increases towards the apex, but on distal segments of the antenna their number stabilizes. It is characteristic of both the T, and to BIB2 sensilla that their numbers are slightly greater on anterior than posterior sides of dorsal sensillar fields, and also greater on posterior than anterior sides of ventral sensillar fields of all antenna1 segments investigated. We assume that the number of olfactory sensilla on the antennae of male beetles coincides with the distribution of strength of olfactory signal on the antennae of beetles orientating in an odour plume. The distribution patterns of T, and B,B, sensilla of the male A. obscurus can be related to some behavioural peculiarities of olfactory orientation (walking or flying and vibrating of the antennae). 0 1997 Elsevier Science Ltd

Index descriptors (in addition to those in the title): Antenna1 morphology; olfactory orientation; SEM

INTRODUCTION

The click beetle Agriotes obscurus is a widespread pest, whose larvae live in the soil and damage the subterranean parts of several agricultural and horticultural crops. The volatile sex pheromone components of female A. obscurus

have been identified (Borg-Karlson et al., 1988; Merivee and Erm, 1993). The use of synthetic sex pheromones for the control of click beetles seems to be promising. It is, therefore, important to understand the chemoreception of these insects.

In insect antenna1 sensilla, structure-function relation- ships are quite well understood. These sensilla show the greatest wealth of structural adaptations (Zacharuk, 1980; Altner and Prillinger, 1980; Keil and Steinbrecht, 1984; Zacharuk, 1985). It is probable that the location of the sensilla on the antenna also has adaptive value and is, therefore, closely correlated with the corresponding behavioural peculiarities in different insect species (flying, walking, vibration of antennae, etc.).

With the exception of previous morphological studies on the sense organs of click beetle larvae (Zacharuk, 1962; Bellamy and Zacharuk, 1976; Zacharuk et al., 1977), very little research has been carried out on the antenna1 sense organs of adult click beetles. The typology and top- ography of the antenna1 sensilla of A. obscurus, however, have been described using a scanning electron microscope

*Author to whom correspondence should be addressed.

(topographic maps of antenna1 sensilla are given) (Meri- vee, 1992). However, the work was based on micro- photographs taken of only one male and female beetle’s antenna and does not allow us to analyse statistically the distribution of sensilla in different areas of the antenna.

The aim of the present morphological study is to show some possible correlation between the distribution of olfactory antenna1 sensilla and the behaviour connected with olfactory orientation in the male click beetles A.

obscurus.

MATERIAL AND METHODS The newly emerged adults of the male click beetles, A. obscurus were

collected from fields in south Estonia during August and September 1994. For examination with the scanning electron microscope (SEM) the whole antennae of freshly killed specimens were removed from the head, cleaned by gentle sonification in distilled water and 70% ethanol for 1 min, attached vertically to aluminium stubs and air-dried before being coated with gold in a vacuum evaporator.

Observations and micrographs were made in a “BS-301” (Tesla) SEM at 1618 kV. The outer surface of the 410th segments of 5 male antennae was microphotographed from 4-6 sides at magnification of 700-2500 x Altogether, a little more than 1000 micrographs were made. Sensillar types, numbers and locations were determined from SEM montage micrographs. In typology of sensilla, the terminology by E. Merivee (1992) has been used. Trichoid T, and Tz sensilla, basiconic B,, B,, and B, sensilla, and dome-shaped D sensilla were studied; B, and B2 sensilla were treated together as one type, B,B2, because due to the presence of intermediate types of sensilla differentiation between them on the micrographs is not always possible.

For depicting the distribution of the number of sensilla, the exterior surface of the antenna1 segments was divided into 4 areas: dorso-

75

76 E. Merivee et al

anterior (DA); dorso-posterior (DP): ventro-anterior (VA); and ventro- were carried out according to the general linear models procedut

posterior (VP) areas (Fig. 1A). the SAS (SAS Institute Inc., 1988).

The number of sensilla for each type was ascertained separately in each area, which made it possible to describe the distribution of the number of sensilla on the antennae of an insect in greater detail than earlier (Merivee, 1992). However, the data about the number of sensilla RESULTS are uneven for several reasons (e.g. because of breakage during the preparation of the antenna), which has been taken into account in statistical analysis. As data was scarce, the total number of sensilla N General structure ofmale beetle antennae on the investigated antenna1 segments were grouped into the proximal part of the antennae NA=N,+N5+N,+N, and distal part

The serrate flagellar antennae of male A. obscwus h

NB=N,+N,+N,,, where N,, N,, , N,, stand for the number of a length of 2.7-3.9mm and consist of 2 basal segme

sensilla on corresponding segment. The statistical analyses (ANOVA) the scape (Fig. 1 B), and pedicel (Fig. 1C); and of

Fig. 1 area c of the SPJ =

(C) PC scape

(A) If the segn

joint :dicel and

D=

Top view of a left flagellar segment of male A. obscurus. DA= dorso-anterior area of the segment, DP=dorso-posterior segment, DVS =dorso-ventral surface of the segment, VP=ventro-posterior area of the segment, VA =ventro-anterior area lent, VS = vibration plane of the flagellum. (B) Scape of the antenna of male A. obscurus. D = dorsal side of the segment, between scape and pedicel, V=ventral side of the segment. The location of some trichoid T, sensilla is shown. Bar= IOOpm. of the antenna of male A. obscurus. D = dorsal side of the segment, V =ventral side of the segment, SPJ =joint between

pedicel. The location of some T, sensilla is shown. Bar=50pm. (D) Third segment of the antenna of male A. obscurus. : dorsal side of the segment, V = ventral side of the segment. The location of some T, sensilla is shown. Bar = 50 pm.

.e of

ave nts:

the

Distribution of Olfactory Antenna1 Sensilla in Male Click Beetle 71

flagellum composed of 9 segments (Fig. ID, 2A-D). Antenna1 segments 4-l 1 have a small dorsal and a major flattened ventral extension in their distal parts. The trichoid T, sensilla (Fig. 2C, D, 3A), basiconic B,B2 sensilla (Fig. 3B), basiconic B, sensilla (Fig. 3A) and dome- shaped D sensilla (Fig. 3C) occur on only these extensions of the flagellar segments, forming dorsal and ventral sensillar fields clearly differentiated from each other, but fused only on the apical segment. Some basiconic B,B, sensilla may occur also on the 3rd antenna1 segment. The location of T, sensilla is not strictly connected with dorsal and ventral extensions of flagellar segments; they are often situated outside of the extensions of the flagellar segments.

The positions of the antennae and those of the sensillar fields located on the flagellar segments are correlated with

the behaviour of the beetle. The antennae of a resting or disturbed beetle are pressed closely against the lower parts of the head, thorax and abdomen. During loco- motion, the antennae of a beetle are directed forward and/or up at nearly 90”. In the case of raised antennae, the dorsal fields of sensilla of antenna1 segments are directed backwards and ventral fields of sensilla forwards; the anterior side of the antennae is located medially and the posterior side laterally.

The movements of the antennae depend only on the movements of the scape and pedicel. The joint at the scape and head allows the scape to move only forward and backward. and to a slight degree also up and down. The joint at the scape and pedicel allows the pedicel to move strictly up and down in one plane. The joint between the pedicel and the first &agellar segment is rigid

Fig. 2. (A) Fourth segment of the antenna of male A. obscurus. D=dorsal side of the segment, V=ventral side of the segment. The

location of some T, sensilla is shown. Bar=50pm. (B) Seventh segment of the antenna of male A. obscurus. D=dorsal side of the segment, V=ventral side of the segment. The location of some T, sensilla is shown. Bar= 50pm. (C) Ninth segment of the antenna of male A. obscurus. D=dorsal side of the segment, V=ventral side of the segment. Some trichoid T, and T2 sensilla are shown. Bar = 50 pm. (D) The apical eleventh segment of the antenna of male A. obscurus. D = dorsal side of the segment, V = ventral side of

the segment. The location of some T, and T2 sensilla is shown. Bar = 50 pm.

E. Merivee et al.

Fig. 3. (A) Detail of the eleventh antenna1 segment of A. obscurus. C,, B, and T2 morphological types of sensilla. Bar= 15 pm. (B) Detail of the ventral area of the ninth antenna1 segment of A. obscurus. B,B,, C, and T, morphological types of sensilla. Bar= 10pm.

(C) Dome-shaped D sensilla on the apical eleventh antenna1 segment of A. obscurus. Bar = 5 pm.

(Fig. 1C). Joints between the other flagellar segments are elastic, which enables them to move in relation to one another (Fig. lD, 2A-C). The movement of the flagellar segments are passive in relation to one another.

Distribution of the number of trichoid T, sensilla on 4-10th an tennal segments

T, sensilla are big (5685 pm in length and 3.44.6 pm dia near the base), nearly straight or slightly curved hairs, protruding at right-angles from the antenna (Fig. lC, D, 2A-D, 3B). These sensilla have longitudinal furrows along their shaft and a rounded tip. In some cases, it is possible to see a certain discontinuity at the tip, which may be the location of a pore. They have a large (8.3- 10.5pm dia) low cuticular collar surrounding the hair base. These sensilla do not move at their bases when touched.

The number of Ti sensilla is significantly greater in the ventral fields than in the dorsal fields both in the proximal and distal parts of the antenna as well as on the entire antenna (PcO.05). The beetles have a small number of T, sensilla on every segment of the antenna, and they are arranged in a circle around the distal part of each segment. Male beetles have a small number of T, sensilla

on every segment of the antenna, and they are arranged in a circle around the distal part of each segment. As a rule, the number of T, sensilla on the 410th segments is 7, in some cases it may be 6 or 8. In the ventral area of the antenna1 segments, there are 4 T, sensilla, 2 on both sides. In the dorsal area of the segments, there are 2 T, sensilla on the anterior side and 1 sensillum on the posterior side of the antenna. Exceptions from that rule are rare. Mean numbers of T, sensilla in different areas of the antenna1 segments are shown in Fig. 4.

Distribution of the number of trichoid T2 sensilla on 4-10th antenna1 segments

T2 sensilla are medium-sized (21-34pm length, 1.8- 2.5pm dia near the base), slender, straight and blunt- tipped hairs, protruding at right-angles from the antenna (Fig. 2C, D, 3A). The base of the hair has longitudinal furrows. The cuticular basal collar (3.4-5.2 pm dia) is low and inconspicuous; sometimes it is lacking altogether.

T2 sensilla are located without exception only on the dorsal and ventral sensillar fields of the segments. Their number in the ventral fields is remarkably greater than in the dorsal fields, both in the proximal and distal parts of the antenna as well as on the whole antenna (PcO.05).


A A 6

_m N = 3-5 F5

0’ . 4 5 6 7 a 9 10

Number of antenna1 segment

B

___c-- _ +- _ _ ____*__c___ VP v VA

6 7 a Number of antenna1 segment

C g 31 E N = 3-5

I B 2- $

DA

g, c__-c-_~_--~--_._ --.--__. DP

c

4 5 6 7 a 9 10 Number of antenna1 segment

Fig. 4. The distribution of the number of trichoid T, sensilla on the antenna1 segments in male click beetle, A. obscurus. (A) Ventral (VFS) and dorsal (DFS) fields, (B) anterior (VA) and posterior (VP) sides of ventral fields, (C) anterior (DA) and posterior (DP) sides of dorsal

fields.

In the proximal parts of the antennae in ventral fields, the mean number of these sensilla increases from 18.3 on the 4th segment up to 65.9 on the 8th segment. On the 8- 10th segments their number becomes more or less stable, forming a “plateau” on the diagram (Fig. 5). In the dorsal fields, there are no T, sensilla on the 45th antenna1 segments; starting from the 6th segment, their mean number increases linearly towards the apex of the antenna, being 7.6 on the 6th segment and increasing to 50.4 on the 10th segment (Fig. 5A).

Analysing the distribution of T2 sensilla on each sensillar field separately, we found that their number is a little greater on the posterior sides of ventral sensillar fields than on the anterior sides (PcO.05) (Fig. 5B) and also greater on the anterior sides of dorsal fields than on the posterior sides (PcO.05) (Fig. SC). However, if we treat ventral and dorsal fields together, then the numbers of Tz sensilla on the anterior and posterior sides of the antennae differ statistically significantly only in the proxi-

VFS

DFS

4 5 s i s 9 1’0 Number of antenna1 segment

B

VP

VA

5 6 7 a 9 10 Number of antenna1 segment

30 m I 1 N=3-5

4 5 6 7 a 9 IO Number of antenna1 segment

Fig. 5. The distribution of the number of trichoid T, sensilla on the antenna1 segments in male click beetle A. obscurus. (A) Ventral (VFS) and dorsal (DFS) fields, (B) anterior (VA) and posterior (VP) sides of ventral fields, (C) anterior (DA) and posterior (DP) sides of dorsal

fields.

ma1 part of the antenna (PcO.05) but not in the distal part (P>O.O5) or on the entire antenna (P>O.O5).

Distribution of the number of basiconic B,B2 sensilla on the 4-10th antenna/ segments

The basiconic BIB, sensilla are 10.8-18.8 pm long with a basal diameter of 1.7-2.7 pm, having slightly curved pegs with rounded tips. The diameter of the raised basal cuticular collar ranges from 5.0 to 6.7 pm (Fig. 3B).

Basiconic BIB, sensilla also occur only in the dorsal and ventral fields of the antenna1 segments, whereby their number on the ventral fields is significantly larger than in the dorsal fields (PcO.05). The mean number of these sensilla in ventral fields is 44.8-55.3, forming a “high plateau” on the diagram. In dorsal fields the mean number of BIB2 sensilla is increasing nearly linearly towards the apex of the antenna, starting from 4.3 sensilla on the 4th segment and reaching 37 sensilla on the 10th segment (Fig. 6A).

Analysing the distribution of BIB2 sensilla on each

80 E. Merivee et al.

A B, sensilla almost always occur on ventral fields of the 00

1 N=3-5 antenna1 segments, only very seldom can they be found on dorsal fields (Fig. 7A, C). The differences in the num-

j-L VFS .

bers of these sensilla are statistically significant in ventral and dorsal fields (PcO.05). Their number is small: 224

c-- -+ DFS

sensilla per segment. These sensilla are, almost without

/ exception, located on the posterior sides of ventral fields ,

,c-_*’ of the antenna1 segments (P < 0.05) (Fig. 7B). ,

*cc *_-a r-

4 5 6 7 0 9 10 Distribution of the number of dome-shaped D sensilla on

Number of antenna1 segment the 4-10th antenna1 segments

B The circular cuticular collar of D sensilla forms a high

rounded dome (height ranging from 4.5 to 5.8pm and N=3-5 diameter from 6.7 to 7.5 cm) from which the round tip of

__*--*- the peg protrudes only l-2 pm (Fig. 3C). The number of D sensilla on male A. obscurus’ antennae is l-3 on the 4- 10th segments; they never occur outside dorsal and ventral fields.

The numbers of D sensilla differ significantly in ventral

4 5 6 7 6 9 10 and dorsal fields both on the proximal and distal parts of Number of antenna1 segment the antenna (PcO.05) but not on the whole antenna

30 1 C

(P > 0.05). Their mean number in ventral fields decreases towards the apex of the antenna from 2.3 on the 4th

0’ 5

4 5 6 7 8 9 10 E’

Number of antenna1 segment 2

o_ c__-)---*-_-*---*- _,a----. DFS

Fig. 6. The distribution of the number of basiconic B,B, sensilla on the 4 5 6 7 6 9 10

antenna1 segments in male click beetle A. obscurus. (A) Ventral (VFS) Number of antennal segment

and dorsal (DFS) fields, (B) anterior (VA) and posterior (VP) sides of ventral fields, (C) anterior (DA) and posterior (DP) sides of dorsal B

fields.

sensillar field separately, we found that their number is slightly larger on the posterior surfaces of ventral sensillar

E 1 fields of the antenna1 segments (PcO.05) (Fig. 6B) and s

a on the anterior surfaces of dorsal fields (PcO.05) (Fig. 6C). If, however, we treat ventral and dorsal fields toge-

0 . VA

4 5 6 7 6 9 10 Number of antenna1 segment

ther, then the numbers of B,B, sensilla on the anterior and posterior surfaces of the antennae are not significantly C different both in the proximal and distal parts of the f’l Nz3-5

antenna as well as on the whole antenna (P > 0.05). B 0 I ‘s

Distribution of the number of basiconic B, sensilla on the ! 4-10th antenna1 segments

2

Sharp-tipped basiconic B, sensilla range in length from 5 SO-

7.0 to 9.1 pm and have basal diameters of 2.0-2.9 pm. 6 7 6

Number of antennal segment

The tip is slightly clavate and in some cases the distal Fig. 7. The distribution of the number of basiconic B, sensilla on the

longitudinal grooves can be seen on micrographs. These antenna1 segments in male click beetle A. obscurus. (A) Ventral (VFS)

pegs arise from a raised basal collar, with diameters rang- and dorsal (DFS) fields, (B) anterior (VA) and posterior (VP) sides of

ing from 5.3 to 7.4pm (Fig. 3A). ventral fields, (C) anterior (DA) and posterior (DP) sides of dorsal

fields.

A 4,


N=3-5

DFS

OJ VFS


I3 3

1 N = 3-5

m I <

2 . P2 \ \ z 2

b---a \

E \

Zl- \ \

5

f b-,_

. . .

. w---4 VP

o- l . VA

4 5 6 7 a 9 10 Number of antennal segment


Fig. 8. The distribution of the number of dome-shaped D sensilla on the antenna1 segments in male click beetle A. obscurus. (A) Ventral (VFS) and dorsal (DFS) fields, (B) anterior (VA) and posterior (VP) sides of ventral fields, (C) anterior (DA) and posterior (DP) sides of

dorsal fields.

segment to 0.2 on the 10th segment. In dorsal fields, their mean number increases towards the apex of the antenna, starting with 0.3 on the 4th segment, to 1.6 on the 10th segment (maximal mean number 1.8 was found on the 9th segment) (Fig. 8A). Analysing the distribution of the number of D sensilla in each field separately, we found that on both ventral and dorsal fields D sensilla occurred without exception on only the posterior sides of the antenna1 segments (Fig. 8B, C).

DISCUSSION

Statistical analysis showed that great differences occur in the distribution of antenna1 T,, Tz, B,B2, B,, and D sensilla in the male click beetle A. obscurus.

T, sensilla have been shown by means of electrophysiological methods to be contact chemoreceptors in male A. obscurus (E. Merivee, unpublished data). The

nearly uniform number of T, sensilla on every flagellar segment and their sparse arrangement in a circle around the distal part of each flagellar segment is characteristic not only of the male beetles of A. obscurus, but also of Curculionidae (Mustaparta, 1973, 1975), Cerambycidae (Dyer and Seabrook, 1975; Dai and Honda, 1990) and Anobidae (Okada et al., 1992). Such an arrangement of contact chemoreceptors, extending over others and protruding at right-angles from the antenna, guarantees that any region of the flagellum in contact with the substrate will facilitate contact of chemoreceptors with that substrate.

The number of trichoid T, sensilla on one antenna of male A. obscurus is more than 700 (Merivee, 1992). They have been shown by means of electrophysiological methods to be pheromone receptors (E. Merivee, unpublished data).

For interpreting the distribution pattern of olfactory sensilla in the male A. obscurus, we follow SkirkeviEius and Boda (1975), who showed that the wind pressure, the number of odour molecule hits and the number of corresponding olfactory receptors in different regions of the antenna of a bee flying in an odour plume, are in positive correlation with one another. The adults of A. obscurus move mainly on the ground and in the grass and do not fly much, especially when the temperature does not exceed 21°C (Brian, 1947). Also, male beetles orientating to a sex pheromone source walk on substrate (Orlov and Ismailov, 1986), at the same time vibrating their antennae energetically up and down (E. Merivee, unpublished data). The moving speed of the antennae in relation to the air can be projected as a sum of 3 vectors: (1) the speed of movement of a beetle in relation to the ground; (2) the speed of the moving antennae in relation to the air, due to the vibration of antennae; and (3) the speed of the wind. The influence of these 3 vectors on summary wind pressure in different regions of the antenna is different in the male A. obscurus moving in pheromone plume on the ground against the wind.

Vibration of the antennae up and down can be precisely associated with the location of Tz sensilla on the segments of antenna in 2 (dorsal and ventral) fields, which is characteristic for A. obscurus. It seems that the pheromone receptors of the male A. obscurus are located only in these regions of the antenna where the likelihood of contact with pheromone molecules spreading through the air is the highest.

As the apex of the antenna is moving with a larger amplitude and speed than the basal segments while vibrating, it is expected that the number of olfactory receptors would increase towards the apex of the antenna (Fig. 5A). Owing to the moving speed of the beetle and the wind, it is not surprising that the number of T2 sensilla is greater and their distribution is more even on the ventral surface of the antenna than on the dorsal surface (Fig. 5B, C).

Because of the morphological peculiarity of the joint between the scape and pedicel in A. obscurus (Fig. lB),

82 E. Merivee et al.

the pedicel and, hence, the whole flagellum can be moved up and down in one plane only. The vibrating plane does not coincide with the dorso-ventral surface of the antenna1 segments, but is turned at 45” (Fig. 1A). Hence, the wind pressure resulting from the vibration is not distributed symmetrically between the anterior and posterior sides of the antenna. On the dorsal side of the antenna, the maximal values of the wind pressure should be shifted towards the anterior surface of the antenna, but on the ventral side towards the posterior surface, coinciding again with the distribution of the number of T2 sensilla in related areas (Fig. 6B, C).

The distribution of olfactory pheromone receptors in some other Coleoptera whose location of antenna1 sensilla has been investigated, is quite different from that of male A. obscurus. These sensilla are restricted to the 2 constriction bands and the distal ring on the antenna1 club of Hylobius abietis (Curculionidae) (Mustaparta, 1973, 1975). Abundant olfactory receptors, pheromone receptors included, are located without exception on the anterior face of the antenna1 club in Scolytidae (Payne et al., 1973; Hallberg, 1982; Faucheux, 1989, 1994). In Semiadalia undecimnotata, abundant trichoid sensilla are present only on the 2 terminal segments of the antenna (Jourdan et al., 1995). In Lasioderma serricorne (Ano- biidae), trichoid sensilla were not found (Okada et al., 1992).

The total number of B,B, sensilla on one antenna of a male A. obscurus reaches over 600. In this case, we are probably also dealing with olfactory receptors (Merivee, 1992). Similar abundant basiconic sensilla are found in nearly every insect species investigated. The receptors in these sensilla usually have broad, sometimes overlapping reaction spectra, apparently involved in food and/or habitat finding, and selection is observed (Schneider, 1964; Keil and Steinbrecht, 1984; Zacharuk, 1985). Therefore, habitat and plant food selection may also be hypothesized for basiconic B,BI sensilla of A. obscurus.

The distribution pattern of BLBZ sensilla on antenna1 segments of the male A. obscurus is quite similar to the distribution of T2 sensilla: (1) they are also located on flagellar segments in dorsal and ventral fields; (2) their number in dorsal fields of flagellar segments is also increasing towards the apex of the antenna nearly linearly (Fig. 6C); (3) in dorsal fields their number is also significally greater on the anterior surface of flagellar segments (Fig. 6C); and (4) in ventral fields their number is also significally greater on the posterior surface of flagellar segments (Fig. 6B). This explains why it may be assumed that in the perception of odours belonging to the reaction spectrum of BIB2 sensilla, the vibration of the antennae also plays an important role.

Because there is significant difference in the distribution of T2 and BIB, antenna1 sensilla in the male A. obscurus, it must be emphasized that as the mean number of Tz sensilla on ventral sensillar fields increased rapidly from 18.3 to 65.9 from the 4th to 8th segments of antenna,

stabilizing on the 8-10th segments, the mean number of BIB2 sensilla on the 410th segments of the antenna on the ventral sensillar fields, varies only slightly (44.8-55.3), forming a “high plateau” in the diagram (Fig. 6A). The location of B,B, sensilla in ventral fields of the antenna1 segments as a “high plateau” permits us to assume that these sensilla are also important in the behaviour of the beetles when they are flying (e.g. finding biotope or food plants by their smell).

The distribution of similar basiconic sensilla in some other Coleoptera is quite different from that of male A. obscurus. They are located in belts on the anterior face of the antenna1 club in Scolytids (Payne et al., 1973; Hallberg, 1982; Faucheux, 1989, 1994), or in 2 constriction bands on the antenna1 club in Hylobius abietis (Mustaparta, 1973). In Monochamus, except for the apical segment, the majority of the abundant sensilla basiconica are confined to 2 sensory fields on the medial side of each segment (Dyer and Seabrook, 1975). In Psacothea hilaris, abundant basiconic sensilla are distributed nearly uniformly on ventral surfaces of the 2nd to 9th fla- gellomeres (Dai and Honda, 1990). In Semiadalia undecimnotata, few basiconic sensilla are located only on distal 2 segments of the antenna (Jourdan et al., 1995). In Lasi- oderma serricorne, the total number of basiconic sensilla per male antenna is about 700, and they are distributed nearly uniformly over the distal region of 410th antenna1 segments (Okada et al., 1992). In Alticinae, abundant long sensilla basiconica are restricted to the 46th distal antenna1 segments (Ritcey and McIver, 1990). “Long sensilla basiconica I” tend to be concentrated on a pro- tuberance of the disto-ventral margin of the sub-terminal 45th segments at the same time as “long sensilla basiconica II” occur over the entire area of each segment.

Sensory organs with an external morphology similar to that of B, sensilla of A. obscurus, were thought to be olfactory receptors (Dickens and Payne, 1978; Fischer and Kogan, 1986) or thermoreceptors (Altner, 1977; Altner et al., 1977) in other beetles. Without additional ultrastructural and/or electrophysiological data, their specific function in A. obscurus cannot be determined. Therefore, it is difficult to interpret their peculiar distribution at this time.

D sensilla resemble in their shape, the rare “short sensilla basiconica I” of flea beetles (Chrysomelidae) (Ritcey and McIver, 1990), and the “conical pegs” of Psacothea hiZaris (Cerambycidae) (Dai and Honda, 1990). These sensilla have been identified as hygro- or thermoreceptors in several insects (Zacharuk, 1985). Further study is needed for final determination of their specific function in A. obscurus.

CONCLUSIONS 1. The distribution pattern of olfactory T2 and B,B, sen-

silla on the antennae of the male beetle A. obscurus differs significantly from the distribution patterns of olfactory sensilla of other beetles investigated.


2. The specific distribution patterns of T2 and BIB2 sensilla on the antennae of the male A. obscurus can be related to the peculiarities of the beetles’ olfactory orientation (walking or flying, and vibrating of antennae).

Acknowledgements-We thank Dr Tdnu Mols (Institute of Zoology and Botany, Estonia) for his help in statistical analysis of data and MS Reesi Lepa for translating the manuscript into English.

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Distribution of olfactory and some other antennal sensilla in the male click beetle Agriotes...

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