BY DR. JOSE PALU-AY DACUDAO
EVERYONE has encountered insects. A million species of insects have been identified, and the number keeps on rising as new species are discovered or defined each year. That’s about half the total of known animal species. In other words, half of all animal species are insects. That’s just a minimum estimate, the number of insect species may number more that 6 million; it’s just that biologists haven’t discovered or identified them yet. They are characterized by a segmented body. They have a head, thorax, and abdomen. As adults they typically have three pairs of legs, one pair of antennae.
What are the insects of the seas?
Easy enough to answer. Crustaceans. Yes, your favorite culinary crab and lobster, shrimp and prawn. Or so it was traditionally thought.
The similarities between insects and crustaceans have been known for hundreds, probably thousands of years. They are so obviously covered with a chitinous exoskeleton (they do not have an internal skeleton), have segmented bodies and jointed limbs.
In that regard, they are also similar to other creatures with the same characteristics, the arthropods. Their common ancestor was characterized by a body made up of multiple segments, each segment bearing a pair of jointed limbs. It probably resembled a worm-like annelid, and annelids (earthworms, etc.) are also segmented. In the course of eras spanning hundreds of millions of years, the segments and limbs evolved into structures that are utilized for various important functions, such as respiration, feeding (including jaws), reproduction, locomotion, or may have gotten lost. Thus, we now get the three traditional subphyla of Phylum Arthropoda – the chelicerata (includes spiders, scorpions, ticks, mites, and horseshoe crabs, characterized by a mouthpart called chelicera); the crustacea (almost all aquatic, freshwater and saltwater, and possessing ten limbs, three main body segments, and two pairs of antennae); and the unirama (centipedes, millipedes, and insects).
As with many traditional taxons, the advent of phylogenetics, which incorporates genes in classifying living organisms, this classification has undergone a revision. A phylogenetic 2017 study Schwentner, et al concluded that insects and the larger cade they belong to, the hexapods, are deeply nested among crustaceans. Several other studies support this.
Land plants first began to grow on land during the Ordovician Period, the second division of the Paleozoic Era, about 488 million to 444 million years ago. Worm-like burrowing animals followed suit, but it isn’t clear what they were. The abundant vegetation offered a whole lot of potential niches just waiting to be occupied by intrepid animals from the sea. The first arthropod to come on land was an arachnid that resembled a scorpion, but insects hopped onto land just a few million years later. This happened in the Silurian Period, the third division of the Paleozoic Era, about 444 million to 416 million years ago. Pretty soon, insects and other arthropods dominated the terrestrial niches, feeding on the abundant vegetation, and on one another.
Fortunately for us, arthropods lack an internal skeleton. They can grow only by molting their chitinous exoskeleton repeatedly. Exoskeletons grow weaker as size increases. Furthermore, the mostly passive tracheal system (a system of tubes) of respiration in insects limit the amount of oxygenated air that could reach the deepest recesses of their bodies. That ability to pump in air, obtain oxygen, and deliver O2 to tissues far away from an animal’s surface requires complex systems of respiration and circulation, including true lungs and hearts.
Thus, these early land arthropods were followed by four legged tetrapods (at that time amphibians) during the Carboniferous period, the fifth division of the Paleozoic Era, about 359 million to 299 million years ago. They evolved from a kind of ancient lobed-finned fish, represented today by the famous living fossil the Coelacanth. With their complex systems of respiration and circulation, the early amphibians and reptiles could grow much larger than arthropods. The abundance of insects offered a smorgasbord for them.
Later also in the Carboniferous, some of these early amphibians evolved the amnion. This covering could protect their eggs from dehydration, and thus they could now lay them on dry land. The layperson’s term for the first amniotes would be reptiles. These reptiles could venture deeper into the land, eating yummy insects. All modern reptiles, birds and mammals possess an amnion. In the case of mammals that give live birth, it’s one of the coverings of babies that burst during birth.
(It must be mentioned that laying eggs characterized the most ancient of mammals 200 million years ago, derived from their synapsid ancestors. Amazingly mammals that lay eggs are still extant. They are the monotremes, represented today by platypuses and echidnas. Or maybe from one perspective just platypuses, as a 2009 multigene study by Phillips et al has found out that echidnas are derived from platypuses probably around 19 million years ago. It’s another fascinating story.)
It’s strange that early amniotes found insects yummy. While we modern humans, who are also amniotes do not (there are exceptions), but find crustacean crabs, lobsters, prawn, and shrimp delicious.
After all insects are crustaceans that went to land./PN
