The Phasmatodea Order is represented by terrestrial insects, phytophagous and wide distribution. About 200 species have been described in Brazil as Cladomorphus phyllinus. The aim of this study was to describe the biology as well as measure population parameters. The specimens were kept in cages with temperature and natural photoperiod. We used 22 animals that were fed with leaves of guava, Psidium guajava Linnaeus. For the construction of the dynamic life table, the following parameters were used: specific survival by age group; survivorship or survival compared to live-born animals; specific fertility by age group; specific mortality by age group and life expectancy. Initially, the cohort of animals was monitored weekly, for a total period of 55 weeks. However, to facilitate data tabulation, they were grouped into age groups. The cohort was then divided into 11 periods of 35 days each; thus totaling 385 days. The survival curve, constructed with the survivorship data, indicate to a curve of type II, where the mortality rate is relatively constant throughout life. The animals began to lay eggs at the 23rd week. The period with the highest fecundity was t5 with 93 eggs per female. With the life table, the population projection was performed in future time intervals. In the time t = 291, the λ value stabilizes at 2.0747 and the population reaches stable age distribution. The data indicate high biotic potential for the species, in contrast to the density of data in the field, when the group is usually found in low densities, probably indicating low effectiveness of camouflage combined with the fragility of the animals against environmental adversities.

Keywords: biodiversity, survival curve, phasmids, cohort table

The Phasmatodea Order is represented by terrestrial insects, that are nocturnal, phytophagous and distribution covering temperate and tropical ecosystems.¹ More than 3.000 species have been cataloged, and the greater diversity of the group is concentrated in the tropics.² These insects are found on trees and resemble twigs, branches, leaves, and lichens.³ The Insects that belong to this order are popularly known as stick-insects.^2,4

About 200 species have been described in Brazil, and for decades, Carausius morosus Sinéty, 1901 was a much used as research animal. Recently other species have also been created in the laboratory for different purposes.^3,4 Cladomorphus phyllinus Gray, 1835 shares with the others the habit of living on plants and feed exclusively on leaves and buds.^5,6

Cladomorphus phyllinus is polyphagous and widely distributed throughout northeast of Argentina, southern Amazon, eastern Brazil including Mata Atlântica and nearby regions.^5,7,8

For construction of the cohort life table or dynamic life table is necessary the number of individuals in every age group, the probability of survival and fertility rate. The life table expresses the interaction between individuals and their environment by providing change information on population size, essential for understanding the population dynamics.^9–11

The objective of the present study was to describe aspects of the biology and life history of the species Cladomorphus phyllinus,⁵ as well as to measure population parameters.

The study was conducted in laboratory of Ecology "Nico Nieser", in the Federal University of Triângulo Mineiro. 22 animals of the species Cladomorphus phyllinus were used. They were kept in a cage with dimensions of 80x75x45 cm, with wooden frames and nylon screen. Testimony exemplaries were deposited in PELL Collection in Uberaba/Minas Gerais (Figure 1).

Figure 1 Testimony exemplaries (female on the right, male on the left) deposited in PELL Collection in Uberaba/MG.

The individuals were fed in guava leaves Psidium guajava Linnaeus, counted every seven days, and the number of eggs was measured (Figure 2).

Figure 2 Specimens of Cladomorphus phyllinus Gray, 1835, bred at the Nico Nieser Animal Facility at the Federal University of Triângulo Mineiro, in Uberaba/Minas Gerais. A female on the right and, on the left, two males and a female.

The following parameters were recorded: survival Sx, defined as the proportion of subjects at age x surviving to age x + 1 (than Sx = nx + 1/nx); lx or survivorship, which is probability of survival over time intervals or survival of newborn to age x, calculated as lx = nx/n₀. By definition all born alive, then lx₀ = 1. The proportion of infants at age 1 is the probability of surviving from age 0 to age 1; than lx₁ = S₀.

By definition, bx is fecundity, or the number of offspring produced per female per breeding season or age. The mortality rate (mx) is the proportion of individuals dying of age x to age x + 1, on average, or mx = [nx – (nx + 1)]/nx.

The life expectancy of the individual of age x (ex) is calculated based on the formula ex = Tx/nx, with life expectancy for living organisms at the beginning of x time interval. For the calculation of ex is required tx calculation, which is the sum of the average survival per interval or the number of surviving individuals per time interval. This parameter has no biological significance and is only used for the calculation of tx.

The logistical and geometric growth models have analogy in the form of growing populations in the exponential growth phase. Thus, the loge λ = r or λ = er. For λ means the population growth rate between time intervals in geometric growth; but it needs to be considered same number of episodes of birth and death.

The life table expresses the survival and fertility of Cladomorphus phyllinus under laboratory conditions for 11 periods of 35 days, totaling 385 days. Age was represented by number of days in the interval x to the interval x + 1, since the reproduction of this species is continuous (Table 1).

Of the 22 animals alive at tx₀, 18 were alive at 35 days (Tx1). So S₀ and L1 are both equal to 0.81. Of these 22 individuals, 14 reach reproductive age, representing approximately 64% of the population (Table 1).

The mortality rate (mx) has remained relatively constant, with higher rates in periods tx₆ to tx₁₀. In the 23rd week, the animals began laying eggs. In tx₄, the observed rate was 5.25 eggs per female, followed by 93.14 in Tx₅, during which there was the greatest fecundity, 93.143 eggs/female. Life expectancy (ex) ranged from 5.14 in tx₀ to 0.50 at tx₁₀.

The survival curve for Cladomorphus phyllinus is shown in Figure 3. The survival curve is constructed with survivorship data, and can take three different forms.¹⁰ The survival curve observed in this cohort of Cladomorphus phyllinus approaches Type II, with a relatively constant mortality throughout life. This pattern is common for some populations of insects and birds.^12–14 In most species of plants, amphibians, reptiles, and small invertebrates, in which young animals are vulnerable to predation and other risk factors, mortality is higher in the early stages of life, featuring the type III of survival curve.¹³

Figure 3 Survival curve for Cladomorphus phyllinus Gray, 1835, bred at the Nico Nieser Animal Facility, at the Federal University of Triângulo Mineiro, in Uberaba/Minas Gerais, created under laboratory conditions.

As pointed out by Pelli et al.,¹⁰ stable age distribution, corresponds to the period in which the population grows, or decreases, at the same rate. In this condition, the percentage proportion of individuals per age group is maintained. This phenomenon occurs when environmental conditions remain constant for several time intervals. Under these conditions the value of λ stabilizes, and the value of the intrinsic rate of instantaneous growth (r) can be estimated.

With the population projection data, computed from the dynamic life table, and considering the geometric growth model, at time t = 291, the λ value stabilizes at 2.074741.

The range in which the population reached a stable age distribution, that is the proportion of the number of animals in each age group, remain constant over the time intervals, with the population increasing or decreasing.

This will only be possible if environmental conditions remain constant for several time intervals and, therefore, specific mortality and birth rates by age group, also remain constant.¹⁰

In the present study, the females laid an average of 362.56 eggs. This value is slightly lower than that observed by Alvarenga et al.,⁷ when the observed value was 392.3/female. Another difference that draws attention is the longevity of the animals. Alvarenga et al.⁷ report that females can live up to eight months of age, whereas in the present study, females lived up to 385 days, a value much higher than that observed by Alvarenga et al.⁷

To Triatoma sordida,¹⁰ the population has reached a stable age distribution at t = 229, with a value of λ = 2.00546, slightly lower than λ value found by us, indicating high potential growth rate for Cladomorphus phyllinus.

The intrinsic rate of increase of a population may be indicated by r_m. This exponential growth rate is calculated for populations with stable distribution. This would be the exponential rate of increase when the population presents stable age distribution. Naturally stable age distribution is only achieved when the environmental conditions remain constant, thus the population attributes remain constant by age group.

As environmental conditions do not remain constant over time, this stable age distribution is rarely achieved. Therefore, we can work with an approximate value of intrinsic rate of increase, called r_a. For this cohort, the calculated r value was 0.72838.

At time t = 291, when the population reached stable age distribution, the percentage distribution of individuals within the population would be 58.421% of individuals in the time interval tx₀, 23,033% at the interval of time tx₁; 10,480% by tx₂; 4.753% by tx₃; 2.005% by tx₄; 0.828% by tx₅; 0.332% by tx₆; 0.096% by tx₇; 0.039% by tx₈; 0.011% to 0.002% and tx₉ in the time interval tx₁₀, showing that most of the population could be constituted by young insects.

In the laboratory, the species Cladomorphus phyllinus presents a high biotic potential, however, in fauna surveys, this group normally presents low densities.^15–19 In systematic collections, this group presents low densities, probably indicating low effectiveness of camouflage combined with the fragility of the animals in the face of environmental adversities.^15–19

In the present study, Cladomorphus phyllinus showed a high biotic potential, as the λ value stabilized at 2.0747. This value is superior to that of several other species already reported in the literature. It is interesting to note that despite this attribute, the species can be considered rare in nature. Few reports exist of the occurrence of the species. The studied species is very showy, and draws attention for its unique traits, yet it is not reported often.

The authors consider that not only the biotic potential can eventually determine the success of the species, but also other variables such as: pollution, destruction of natural habitats and, in a way, an eventual strategy. No predator will specialize in rare prey. Another hypothesis raised is that the primary line of defense of the species is not effective. These animals camouflage themselves and, therefore, are called leaf and stick insects. However, if the strategy were efficient, it would be able to obtain resources and, at the same time, not be used as a resource.

In conclusion, the authors point out the high biotic potential of the species, associated with its eventual fragility under natural conditions, which reflect in low population densities.

To Prof. Dr. Ângelo Barbosa Monteiro Machado (in memoriam), at the Department of Zoology at the Federal University of Minas Gerais, a great enthusiast of Brazilian entomology. Prof. Ângelo Machado kindly provided the stick insect matrices.

Institute of Biological and Natural Sciences of the Federal University of Triângulo Mineiro.

Authors declare that there is no conflict of interest for the publication of this article.

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