Morphometry and morphology of the antennae of Panstrongylus megistus Burmeister, Rhodnius neglectus Lent, Rhodnius prolixus Stal and Triatoma vitticeps Stal (Hemiptera: Reduviidae)

SYSTEMATICS, MORPHOLOGY AND PHYSIOLOGY

Morphometry and morphology of the antennae of Panstrongylus megistus Burmeister, Rhodnius neglectus Lent, Rhodnius prolixus Stal and Triatoma vitticeps Stal (Hemiptera: Reduviidae)

João A da Rosa; Silvia C M de Freitas; Flávio F Malara; Cláudia S Rocha

Lab de Parasitologia, Faculdade de Ciências Farmacêuticas/Unesp/Araraquara, Rod Araraquara-Jaú km 1, CP 502, 14801-902 Araraquara, SP, Brasil; [e-mail]; [e-mail]; [e-mail]; [e-mail]

ABSTRACT

The length of the four right antennal segments from nymphs and adults of Panstrongylus megistus Burmeister, Rhodnius neglectus Lent, Rhodnius prolixus Stal and Triatoma vitticeps Stal were measured. The length of the antennal segments of the adults of all four species, 4^th and 5^th instars of P. megistus, and 5^th instar of R. neglectus and R. prolixus followed the same pattern: 2^nd>3^rd>4^th>1^st. The pattern of 1^st and 2^nd instars of P. megistus was: 4^th>3^rd>2^nd>1^st. For 3^rd instars of P. megistus, 1^st and 2^nd instars of R. neglectus and R. prolixus they were: 3^rd>4^th>2^nd>1^st. Third and 4^th instars of Rhodnius neglectus and R. prolixus had a pattern of: 3^rd>2^nd>4^th>1^st. Only T. vitticeps showed the same pattern (4^th>3^rd>2^nd>1^st) for all five instars. The morphological study of the second antennal segment by scanning electron microscopy (SEM) disclosed that the first instars of all four species exhibit type I bristles sensillae and one trichobothrium. Another type III bristle and basiconic, campaniform, coeloconic, trichoid sensillae and type I bristle and trichobothria were noted on their fourth instars and adults. Campaniform sensillae were noted only on T. vitticeps adults. Nodules were observed in the joint between 1^st and 2^nd antennal segments of adults of P. megistus and T. vitticeps, but not on R. neglectus and R. prolixus.

Key words: Triatominae, mensuration, electron microscopy, sensilla

The Chagas' disease, which is caused by the flagellate protozoan parasite Trypanosoma cruzi, is widespread in North and South America from Mexico to southern Argentina and Chile, and is still considered a major public health issue in Latin America (WHO 2005). Great advances have been made in the reduction of vectorial and transfusional transmission, with a resulting alleviation in the incidence of Chagas' disease. Yet, it is estimated that a total of 18 million individuals are still infected in 17 Central and Latin American countries. Approximately 93 million people live in the endemic areas, and 200,000 new cases occur annually in these areas (WHO 2005).

Panstrongylus megistus Burmeister was first recorded to transmit T. cruzi in 1909 (Chagas 1909), and is one of the six major species vectoring the Chagas' disease (Silveira 1983, Dias 1993, Brazilian Health Ministry 2005). Rhodnius neglectus Lent plays a secondary role in transmitting the disease; however, it is widespread in the Brazilian states of Bahia, Goiás, Mato Grosso, Maranhão, Minas Gerais, Paraná, Pernambuco and São Paulo (Silveira 1983, Galvão et al 2003). Rhodnius prolixus Stal can be found in 16 Latin American countries and is an important vector of T. cruzi, especially in Venezuela, Colombia and French Guiana (Lent & Wygodzinsky 1979, Galvão et al 2003). Even though Triatoma vitticeps Stal is considered the most important vector in Bahia, Espírito Santo, Minas Gerais and Rio de Janeiro, studies on this species are very scarce (Galvão et al 2003).

Despite the fact that immatures also play an important role in the transmission of the Chagas' disease, there are very few information on their morphology. In an early article presented by Rangel (1979) some morphological aspects of the anatomy of the digestive apparatus of P. megistus nymphs are mentioned and distinct features of nymph instars are presented in other studies (Côrrea 1954, Ramírez-Pérez 1969, Carcavallo et al 1978, Lent & Wygodzinsky 1979,Jimenez & Fuentes 1981, Brewer et al 1981, Gonçalves et al, 1985, Rosa et al 1992a, 1992b, 1995, 1999, 2000, Rosa & Barata 1997, Galvão et al 2005).

Rosa et al (2000), when studying two colonies of Triatoma rubrovaria, perceived that in nymphs of the 1^st and 2^nd instars, the relative lengths of the four antennal segments were in the order 4^th>3^rd>2^nd>1^st, while in nymphs of the 3^rd instar, the order changed to 3^rd>4^th>2^nd>1^st and in nymphs of the 4^th and 5^th instars and male and female adults, it was 2^nd>3^rd>^4th>1^st.

The importance of the study of the morphological traits of triatomine vectors, beyond their physiological importance as reported by Wigglesworth & Gillett (1934), Chaika (1980) and McIver & Siemicki (1984, 1985), has already been emphasized by Lent & Wygodzinsky (1979), who showed that the length pattern of the four antennal segments can also be used in taxonomic analysis, a fact that justifies the importance of this study.

We present a morphometric and morphological evaluation of the antennal segments of nymphs and adults of P. megistus, R. neglectus, R. prolixus and T. vitticeps, by means of stereomicroscopy, scanning electron microscopy, to provide new information for Triatominae taxonomic studies.

Material and Methods

Insects. A total of 120 specimens for each one of the four above-mentioned species were used, 15 for each instar from 1^st to 4^th and 15 for each sex at the 5^th instar and adult stage. Panstrongylus megistus (colony 139) was collected on February 11, 1985 in Santa Maria do Cambucá area, Pernambuco State, Brasil; R. neglectus (colony 16), collected on April 6, 1982 in Pitangueiras area, São Paulo State, Brasil; R. prolixus (colony 14), from Colombia and established in the insectary on March 6, 1982, and T. vitticeps (colony 41) from Minas Gerais State, Brasil, donated by the Faculty of Public Health, USP, on September 8, 1982. Species were identified according to Lent & Wygodzinsky (1979). The insects were kept in the collection of the Triatominae Laboratory in the insectary of the Araraquara Special Health Service (SESA), São Paulo State. All specimens were bloodfed on ducks (Anas platyrhynchos) for a fortnight at 45 to 50 min intervals.

Insect preparation. Specimens were killed by exposure to chloroform for 1-2 min, and had their right antennal segments excised for assessment of their size in dorsal position, as described by Rosa et al (2000).

Measurements. Antennal segments were measured at 800x magnification in a Leica MZ APO stereomicroscope coupled with a CCD high performance COHU camera, and images were analyzed with the Leica Q-Win software. Trichobothria of adults of the four species (10 specimens each) were counted under the Leica MZ APO stereomicroscope.

Morphological procedures. Chloroform killed specimens were sonicated (Thorton), dried at 50ºC for 20 min. For the morphological study were used the second antennal segment of each specimen, which were fixed horizontally to metallic supports. The antennae were sputter-coated with gold in a vaccum vaporizing metallizer (Edwards), using a pressure of 10^-6. After the metallization, the antennal segments were observed and photographed under a scanning electron microscopy (SEM) Topcon - SM-300 Rosa et al (1992b).

Statistical analysis. Data on the size of the antennal segment were analyzed by using the GraphPad InStat version 3.06 package. Size differences among the four antennal segments were tested for significance by ANOVA (One-way Variance Analysis) and analyzed by the post-test Tukey-Kramer multiple comparison method.

Results

Morphometric studies. Measurements of the antennal segments indicated the size of the segments varied among instars and the species studied, with the exception of T. vitticipes that showed the same pattern of size for all antennal segments throughout the immature development (Tables 1, 2 and 3). No differences were observed in the pattern of size of the antennal segments between sexes within the same species (Tables 1, 2, 3).

Multiple pairwise comparisons between species indicated that the length of all four antennal segments of each of the five instars and adults differed between P. megistus and T. vitticeps, P. megistus and R. neglectus, P. megistus and R. prolixus, T. vitticeps and R. neglectus, R. neglectus and R. prolixus, except for the third segment of the 1^st instar of P. megistus and R. neglectus; there were no significant differences between T. vitticeps and R. prolixus (Table 4).

Morphological studies. The second antennal segments of all instars of P. megistus, R. neglectus, R. prolixus and T. vitticeps were examined by SEM, and differences and similarities in the number and type of sensillae were observed (Figs 1a-f).

 

The 1^st instars of P. megistus, R. neglectus, R. prolixus and T. vitticeps displayed type I bristles and one trichobothrium located at the posterior third of the dorsal side of the second antennal segment (Fig 1a).

The 2^nd and 3^rd instars of P. megistus, 2^nd, 3^rd and 4^th of R. neglectus and 2^nd to 5^th instars of R. prolixus and T. vitticeps displayed the same pattern of sensillae as the 1^st instar, plus a few type III bristles (Fig 1f). The 4^th and 5^th instars of P. megistus showed types I and III bristles, coeloconic and trichoid sensillae and one trichobothrium (Fig 1b).

In adult male and females of P. megistus, the second antennal segment showed type I bristles, trichoid and coeloconic sensillae, basiconic sensillae and four to nine trichobothria (Fig 1c). Adult male and female of R. neglectus showed types I and III bristles, coeloconic sensillae and trichobothria (Fig 1d,e). Rhodnius prolixus showed types I and III bristles and trichobothria (Fig 1f). Triatoma vitticeps had types I and III bristles and campaniform and coeloconic sensillae and trichobothria (Fig 1h).

The number of trichobothria on the right and left segments of the antenna of adults varied from two to nine in both sexes for all studied species (Table 5).

Small nodules separating the first and second antennal segments were observed in P. megistus and T. vitticeps, but were absent in R. neglectus and R. prolixus (Figs 1g, i).

Discussion

The origin of the Triatominae is controversial since they probably evolved from predatory Reduviidae. Triatominae are classified as monophyletic according to Usinger (1944), Lent & Wygodzinsky (1979) and Hypsa et al (2002), or polyphyletic (Schofield 1988, Schofield & Dujardin 1999, Paula et al 2005). The combination of anatomical, physiological and ethological factors observed in this group calls for new research on the morphology of species, which could be useful for taxonomy and in the control of Chagas' disease in endemic areas.

The occurrence of small nodules separating the antennal segments of Triatominae has been reported for several other species (Lent & Wygodzinsky 1979, Rosa et al 1999).

Although Lent & Wygodzinsky (1979) reported that in most species of Triatominae the length of the antennal segments decreased from the second to the fourth segment, the size of the segments varied according to the instar in Triatoma rubrovaria (Rosa et al 2000), as observed for all four species investigated in here (Tables 1, 2, 5). In the case of T. vitticeps, changes were observed only for the adult (Tables 1, 5).

Triatominae have hundreds of sensillae on their four antennal segments (Catalá 1994). Twelve different sensillae were identified on the antennal segments of 10 out of the 16 species of Rhodnius studied from laboratory colonies (Catalá & Schofield 1994, Galvão et al 2003).

Trichobothria are a type of sensilla that is longer and thinner than the others and a varied length, with a diameter ranging from 60 µm to 210 µm. They are randomly distributed and may have mechano, thermo and chemoreceptor activity, and are very common to arthropods (Schuh 1975, Catalá & Schofield 1994). The number of trichobothria in Triatominae is also suggested to have a taxonomic value (Lent & Wygodzinsky 1979).

Our observations agree with those of Lent & Wygodzinsky (1979), which indicated the existence of a single trichobothrium situated subapically on the dorsal side of the second antennal segment. By contrast, adult triatomines display several (2-9) trichobothria on the second antennal segment (Table 4).

In adults, Lent & Wygodzinsky (1979) and Catalá & Schofield (1994) studied the number and location of trichobothria in species of Triatominae. The number of trichobothria described by them differs from those recorded in this study. Previous observation of many specimens of the four species studied in here showed that the number of trichobothria vary in the adults of P. megistus (four to eight), R. neglectus (two to seven), R. prolixus (four to nine) and T. vitticeps (three to nine) (Table 3), in contrast to the nymphs.

Another taxonomic contribution made by this study was the finding of campaniform sensillae on the posterior part of the second antennal segment of adults of T. vitticeps, but not found in P. megistus, R. neglectus or R. prolixus (Figs 1a-f). Triatoma vitticeps, differently from the other three species, presented only two patterns of antennal segment lengths, whereas P. megistus, R. neglectus and R. prolixus showed three patterns. These observations will be useful for phylogeny and, together with other studies, may contribute to determining whether the subfamily Triatominae is a polyphyletic or a monophyletic group.

Acknowledgments

We thank Prof Dr. José Maria Soares Barata, responsible for the Triatominae insectary of the School of Public Health (USP), located at the Special Health Service of Araraquara, who donated the specimens utilized. Thanks are also due to Prof Dr José Clóvis do Prado Jr for his revision and suggestions, Prof Dr Mario Cilense who consented to the utilization of SEM, João Luis Molina Gil and João Maurício Nóbrega Filho, who maintain the insect colonies and to Tim Roberts and João Eduardo da Rosa, who reviewed the English text. Financial support came from PIBIC/ CNPq.

Received 13/X/08.

Accepted 11/II/09.

Edited by Eunice Galati - FSP/USP

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Publication Dates

• Publication in this collection
  14 May 2010
• Date of issue
  Apr 2010

History

• Received
  13 Oct 2008
• Accepted
  11 Feb 2009