Indian Insects

Diversity and Science

S Ramani, Prashanth Mohanraj, Yeshwanth HM, S Ramani, Prashanth Mohanraj, Yeshwanth HM

• Éditeur: CRC Press
• Année: 2019
• ISBN:
  9780429590078
• Édition: 1
• Sujet: Biowissenschaften
• Sous-sujet: Ökologie

Insects are the most interesting and diverse group of organisms on earth, many of which are useful as pollinators of crops and wild plants while others are useful as natural enemies keeping pestiferous insects in check. It is important to conserve these insects for our survival and for this the diversity of insect species inhabiting the different ecosystems of our country must be known. The cornerstone to studies of any kind of organismal diversity is their taxonomic identity. Even after over two and half centuries of studies, so little is known of the insect wealth of our country. It has contributions from taxonomists who have been studying Indian insects for long, this book offers up to date information on many important groups of Indian insects seeking to fill the lacuna of a long felt need for a comprehensive work on the taxonomy of Indian insects.

Salient features:

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• Provides an up-to-date taxonomy of major insect groups of India

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• Presents identification keys with illustrations of several important groups of Indian insects

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• Gives a new insight into why insects are so abundant

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• Addresses fundamental questions in mechanoreception and cross kingdom interactions using insects as model systems

Indian Insects: Diversity and Science is a festschrift to Professor C. A. Viraktamath, an insect taxonomist par excellence. It has been designed to cater to the needs of academicians, researchers and students who wish to identify insects collected from local environments and will be an invaluable aid for those working in the areas of systematics, ecology, behaviour, diversity and the conservation of insects.

1	Why Are Insects Abundant? Chance or Design? K. N. Ganeshaiah

CONTENTS

A Quip: God and the Beetles

Why Are Beetles Species Rich?

Are Beetles Uniquely Diverse Compared to Other Groups?

Species Radiation as a Positive Feedback Process: A Hypothesis

Simulating the Process

Results of the Simulation

Returning to the Quip and the Question

Acknowledgements

References

A QUIP: GOD AND THE BEETLES

Some quips live long; so much so that they come to be established almost as eternal truth. One such rare quip is by Haldane on God’s fondness for beetles. Highly impressed by Haldane’s deep insights on biological systems, an elite lady-theologian is believed to have asked him at a post dinner discussion, on any distinct feature Haldane would identify with, or attribute to “God.” Haldane is supposed to have quipped immediately: God must have an inordinate fondness for beetles (Hutchinson 1959).

Though the exact line that Haldane is supposed to have uttered, the context in which he may have said so, and even the veracity of the event, have all been frequently questioned, discussed, and debated (Gould 1993). The implication of the quip per se, that beetles are the most diverse group of organisms seems to be a pervading message not only among the public, but even among biologists. Gould (1993), tracing the veracity of Haldane’s quip stated, “Ultimate meaning must reside in the unparalleled diversity of that group that rarely rivets our attention.” In fact, the quip has been so influential that several workers have repeatedly cited it in different contexts (e.g., Sagan [Anonymous 2002] while taking a jab at creationists), discussed it (Gould 1993; Hardy 2002), and derived inspiration to address its implication (Ganeshaiah 1998). In some sense, owing to this quip, “In evolutionary biologists’ minds, beetles have been basking in the warm glow of God’s favour ever since” (Hardy 2002).

Beetles have become a subject of special attention to biologists, not just because of Haldane’s quip, but due to two of their features: (a) they are the most species rich group, and hence by derivation, (b) they are considered the most diverse group of organisms. Among the 1.82 million described species, beetles with about 400,000 species, constitute the largest group. Though these numbers may vary across the data sets compiled, it remains unquestionable that ~22 percent of all known species are beetles (Ghosh 1996; Ganeshaiah 1998); but see arguments for the greater abundance of Neoptera (Hardy 2002; Mayhew 2002) and Hymenoptera (Forbes et al. 2018) among insects. Thus, if we randomly sample a set of five species from the pool of all known species, almost always one among them would be a beetle. This unique, species rich feature of beetles remains equally strong when extinct groups from the fossils are also considered, suggesting that they have been the most predominant group in evolution.

WHY ARE BEETLES SPECIES RICH?

For Haldane, the God who created this species rich group is clearly natural selection and hence, for evolutionary biologists (or adaptationists), the most obvious fallout of the attention that beetles have gathered is, “why are there so many of them?” Not surprisingly, this has been the subject of several investigations. For instance, Farell (1998) addressed this question using a phylogenetic approach of the feeding behavior of beetles and showed that their high species richness is the result of the enhanced rates of speciation of the group when they shifted from feeding on lower plants to angiosperms. With this, Farell concluded that God’s, “Inordinate fondness [had been] explained” (Farell 1998; Anonymous 2018a).

On the other hand, Smith and Marcot (2015) examined the rate of extinction of beetles in the fossil data and found that their richness is owing to their resistance to extinction. They then concluded that “focusing on … factors that have inhibited beetle extinction…should be examined as important determinants of their great diversity today.” And there are others who have suggested adaptive features such as wing flexion (Mayhew 2002), phytophagy, or complete metamorphosis (see Hardy 2002) as factors driving beetle species richness—none, however, being unequivocal determinants.

However, before addressing the factors causing the abundance of beetles, it may be important to ask whether they are indeed uniquely different in this feature of species loading. Because, only if they are found to be uniquely species rich, does it become meaningful to ask, “why (and how) are they species rich?” On the other hand, if their species richness could emerge merely from a general pattern through which biological diversity gets organized during evolution, then it would be futile and unnecessary to pursue the question. Alternatively, we may have to explore the processes shaping the organization of diversity. In other words, it may be worthwhile to assess whether the observed species richness of beetles is an inevitable consequence of the evolution of hierarchical organization of diversity even before assessing the biological and adaptive features that may have shaped their richness.

ARE BEETLES UNIQUELY DIVERSE COMPARED TO OTHER GROUPS?

Based purely on the numbers of species in different orders of insects, beetles (Coleoptera) do seem to be uniquely species rich. For instance, at the global level, Coleoptera, with 350,000 species, are almost two and a half times richer than their nearest competing order Lepidoptera (142,500 species), and three and a half times richer than the next rich order Hymenoptera (100,000 species) (Ghosh 1996). Thus, as long as we consider these groups as distinct taxonomic entities, it does seem to validate the general belief that beetles are uniquely rich compared to other orders. But when we attempt to assess if there exists a pattern among all the orders of insects in their species richness, the bubble of beetles’ unique richness seems to collapse, as also God’s “inordinate fondness” for them.

When the numbers of species in different orders are plotted against their size rank (on the basis of their species richness) an interesting and a very revealing pattern emerges. Species richness of orders exhibits a non-linear, exponential increase with the order size (Figure 1.1; Y = 146.7 × e^0.238; R² = 0.891; p < 0.001) and Coleoptera, expectedly, occupies the top right corner of the space. This pattern is true for the data on Indian insects also (Figure 1.2; Y = 6.28 × e^0.262; R² = 0.956; p < 0.001). Though, by the manner in which this relation is constructed, the species richness of the order has to increase, there is no immediate reason to expect it to be a non-linear rise; it could have been a simple linear increase (with a constant number of species added for every size rank) or a power function (with decreasing numbers of species added with increase in the order size). The observed exponential increase suggests that the number of species added (rate of increase) increases with the order size; that means the higher the size rank of an order, there are disproportionately higher numbers of species in it. Further, the size rank of the order is not dictated by their phylogenetic relationship, nor by the antiquity of the orders. Rather, there appears to be an evolutionary process driving this relation across orders, a process that seems to be acting independent of the biological features of the orders. In this sense, the positioning of Coleoptera as a uniquely rich group could be merely a consequence of such hidden processes shaping this unique relation not necessarily due to any adaptive features of beetles.

FIGURE 1.1 Relation between species richness of the insect orders with their rank based on the size for the global data. (Data from Ganeshaiah, K.N., Curr. Sci., 74, 656–660, 1998; Ghosh, A.K., Orient. Ins., 30, 1–10, 1996; Romoser, W.S. and Staffalano, J.G., The Science of Entomology, William C. Brown, Ottumwa, IA, 3rd ed., 1994.)

FIGURE 1.2 Relation between species richness of the insect orders with their rank based on the size for the Indian insect data. (Data from Ghosh, A.K., Orient. Ins., 30, 1–10, 1996; and see Ganeshaiah, K.N., Curr. Sci., 74, 656–660, 1998.)

Further, this relation does not seem to be unique to the class Insecta nor is it restricted to the ordinal level within a class. For example, plants (Figure 1.3), mammals (Figure 1.4), and birds (Figure 1.5) also show a similar exponential relation between the species loading of orders with their size rank. And in each such relation there is (and has to be) one order that always has disproportionately more species than its immediate predecessor. The species loading of families also shows a similar exponential increase with size rank (Figure 1.6).

FIGURE 1.3 Relation between the species richness and rank order in mammals.

FIGURE 1.4 Relation between the species richness and rank order in angiosperms.

FIGURE 1.5 Relation between the species richness and rank order in birds.

FIGURE 1.6 Relation between species load of families and their size rank in insects of the world.

Thus, there appears to be a repeating pattern of species loading of taxa with their size rank, and insects are no exception to this relation. Therefore, the apparent species richness of beetles could be explained if one understands the underlying process generating this relationship, found to be consistent across taxonomic groups and along different taxonomic hierarchies.

SPECIES RADIATION AS A POSITIVE FEEDBACK PROCESS: A HYPOTHESIS

Since the observed non-linear relation between species loading of groups and their rank size seems to be a pervading pattern across...