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Differences in the size of eggshells among three Pangstrongylus megistus colonies

João A da Rosa^I; Hernany H G Justino^I; José M S Barata^II

^IDepartamento de Ciências Biológicas da Faculdade de Ciências Farmacêuticas, Universidade Estadual Paulista. Araraquara, SP, Brazil
^IIDepartamento de Epidemiologia da Faculdade de Saúde Pública, Universidade de São Paulo. São Paulo, SP, Brazil

Correspondence

ABSTRACT

The objective of the study was to evaluate the measurements of eggshells from three colonies of P. megistus, two from the lab and one from a sylvatic habitat. Fifty eggshells from each colony were measured with the use of a stereoscopic microscope and the Qwin image analysis system. The statistical analysis was performed using unpaired t-test in INSTAT program. The length, width and diameter of the opercular opening of the eggshells from the sylvatic colony were larger than the two laboratorial colonies of P. megistus.

Keywords: Panstrongylus. Eggshell. Morphometry. Panstrongylus megistus.

INTRODUCTION

Panstrongylus megistus is considered one of the principal vectors of Chagas disease due to its high susceptibility to Trypanosoma cruzi, broad geographic distribution and adaptation to various habitats and hosts. For this reason, it may be found in households and peridomestic habitats in the State of Bahia, in sylvatic or in domestic habitats in the southeastern region, while in the southern region of the country it is predominantly sylvatic (Barbosa et al^2,3 1999, 2001)

P. megistus is a species that colonizes even in residual forests, as Pinto⁵ has observed. The latter has been collecting specimens of P. megistus in experimental chicken coops installed in farms in the municipality of Araraquara, in Sao Paulo State, every trimester since 1980.

The morphology and morphometry of the Triatomine eggs is important from a taxonomical perspective, as demonstrated by Barata¹ (1981) who elaborated a dichotomous key for identifying ten species of Rhodinius.

References to differences in morphometry among sylvatic and laboratory populations of Triatominae (Dujardin et al,⁴ 1999) were found in the bibliography. However, in the literature consulted, it was verified that there are no references to differences in the size of eggs among sylvatic and laboratory Triatomine colonies.

P. megistus is an important vector for Chagas disease in several countries and in various Brazilian States. For this reason, an evaluation of the morphometry of the eggshells of a sylvatic and two laboratory populations of this species was conducted, thus expanding the existing Triatominae population studies (Barbosa et al,^2,3 1999, 2001).

In April, 2000, 22 females and 27 males were collected from a sylvatic colony of P. megistus, established in an experimental chicken coop, located in a residual forest in the Fazenda Caranda in Araraquara, Sao Paulo, Brazil. The adult female specimens were in a phase of oviposition and 567 eggs were collected and maintained in the laboratory. After the eggs hatched, their length, width and diameter, as well as the opercular opening of 50 shells were measured by means of a stereoscopic microscope and the QWin image analysis system.

Fifty eggshells from two laboratory colonies denominated CTA 146 and CTA 150 were also measured according to the same procedure. The CTA 146 colony was collected in the municipality of Boca da Mata, Alagoas and the CTA 150 was collected in Sao Joao da Boa Vista, Sao Paulo. Both colonies have been maintained in the Insetário de Triatominae of the Faculdade de Saúde Pública [Triatominae Insectary of the School of Public Health] since 1985, located in the Serviço Especial de Saúde de Araraquara (SESA) [Araraquara Special Health Service] being allowed to feed, every two weeks, on ducks.

In order to measure the eggshells, these were enclosed between two glass microscope slides held together with double-coated tape. The samples were arranged in such a way as to permit acquisition of the image by means of stereoscopic microcopy. A routine for obtaining measurements was set up in the Leica Qwin image analysis program and the measures were then recorded. Statistical analysis was performed using unpaired t-test in INSTAT program.

The size of the diameters of the opercular opening as well as the width and length of the fifty eggshells from the single sylvatic and the two laboratory colonies (CTA 146 and CTA 150) are displayed in the table below.

Statistical analysis of the measurements indicate that the eggshells from the sylvatic colony of P. megistus were significantly wider (1.40 mm in average) when compared to the laboratory colonies CTA 146 (mean width of 1.30 mm, P<0.05) and CTA 150 (mean width of 1.28 mm, P>0.05) (Table).

Comparing the mean length of the eggshells from the sylvatic colony with the mean length of the eggshells from the CTA 146 colony, no significant difference was found (mean length of the sylvatic colony: 185 mm, P>0.05; mean length of the CTA 146 colony: 1.81mm, P>0.05) (see Table). On the other hand, the mean length of the eggshells from the CTA 150 colony were significantly smaller than those from the sylvatic colony (1.76 mm; P>0.05).

The mean opercular opening diameter of the sylvatic P. megistus eggshells was 0.68 mm, being significantly larger than that of the CTA 146 colony which was 0.65 mm. The eggshells from the CTA 150 colony had a mean opercular opening diameter of 0.67 mm, which was not significantly different from the sylvatic specimens (0.68 mm).

The results of the P. megistus eggshell measurements are relevant for they can be utilized as a parameter to evaluate the differences between domestic and sylvatic colonies.

The fact that the sylvatic colony of P. megistus included in this study presents larger eggshells than the two laboratory colonies is subject to different explanations and interpretations.

Perlowagora-Szumlewicz⁶ (1976) suggested it was possible that the endogenous reproduction of the laboratory colonies, propitiated by a long period of permanence in this environment, could reduce the biotic potential of the Triatominae.

Dujardin et al⁴ (1999), on the other hand, consider that a reduction in the size of the eggs in the laboratory colonies may be due to population density, for competition would lead to the ingestion of a smaller quantity of blood, which would influence the females primordially, leading to changes in oviposition as well as in parameters of morphometry.

Therefore, the results of this study corroborate both Perlowagora-Szumlewicz’s⁶ (1976) and Dujardin et al’s⁴ (1999) hypothesis.

Thus, reduction in the size of eggshells in Triatominae populations may be an indicator of their possible habitat, whether we are considering one or the other of the hypothesis raised to explain this phenomenon.

REFERENCES

1. Barata JMS. Aspectos morfológicos de ovos de Triatominae. Rev Saúde Pública 1981;15:490-542.        

2. Barbosa SE, Diotaiuti L, Soares RPP, Pereira MH. Differences in saliva composition among three Brazilian populations of Panstrongylus megistus (Hemiptera, Reduviidae). Acta Tropica 1999;72:91-8.        

3. Barbosa SE, Soares RPP, Pires HHR, Diotaiuti L. Experimental evidence for a demographic cline in Panstrongylus megistus populations. Mem Inst Oswaldo Cruz 2001;96:773-5.        

4. Dujardin JP, Steindel M, Chavez T, Machane M, Schofield CJ. Changes in the sexual dimorphism of Triatominae in the transition from natural to artificial habitats. Mem Inst Oswaldo Cruz 1999;94:565-9.        

5. Pinto PLS. Circulação e caracterização de Trypanosoma cruzi isolados de mamíferos silvestres capturados no Estado de São Paulo, Brasil [tese de doutorado]. São Paulo: Faculdade de Saúde Pública da Universidade de São Paulo; 2000.        

6. Perlowagora-Szumlewicz A. Laboratory colonies of Triatominae, biology and population dynamics. Washington (DC): PAHO; 1976. p. 63-82. (PAHO Scienfic Publication, 318).        

Correspondence to
João Aristeu da Rosa
Departamento de Ciências Biológicas, UNESP
Rodovia Araraquara-Jaú Km 1
14801-902 Araraquara, SP, Brasil
E-mail: rosaja@fcfar.unesp.br

Received on 5/9/2002.
Reviewed on 20/3/2003.
Approved on 7/5/2003.
Supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp – Process n. 1997/10708, 99/0999524-3)