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Systematic, Classification

and

Identification

 

For study reasons, all living organisms are divided into homogenous groups by using hierarchical categories; the upper category includes all the lower ones. This system was stated for the first time in the book "Systema Naturae" (1758) by Carl von Linné (Carlo Linneo, generally cited with the Latin name Linnaeus). The main categories are: "kingdom" (regnum), phylum, "class" (classis), "order" (ordo), "family" (familia), "genus" and  "species".

 

For example, here it is the classification for the stink bug Nezara viridula.

 

Kingdom: Animalia

Phylum: Arthropoda

Class: Insecta

Order: Heteroptera

Family: Pentatomidae

Genus: Nezara

Species: viridula

 

Generally, we use also intermediate categories such as the "superfamily" which includes two or more families, or the "subfamily" grouping two or more genera. Really, each category is an "abstraction" built on the basis of "characters" shared by more living organisms. 

 

In praxis each living organism is identified by using a couple of names ("binomial nomenclature"), that is by the genus and by the species in its "properly meaning", sometimes a third name is used to identify the "subspecies" (or geographical race). Up today, there is not a definition of "species" having an universal validity: we can go from a "typological" definition, in this meaning the species is "a group of organisms having a minimal and sufficient range of common morphological characters ", to the "biological" definition in which the species is "a group of inter-fertile organisms". The first definition clashes with the existence of "organisms" that, even though had the same morphology or with only insignificant differences, are not "inter-fertile"; the second one, of course, cannot be used for agamic organisms and for extinct organisms.

Generally, it is not easy to identify, that is to individuate the species of the arthropod we are looking at. Sometimes, such as for the butterflies, it is enough to compare our insect with photos found in books; but this is not always valid. Usually we have to look at certain morphological characters that are not always easily visible such as, the veins in the wings or the number of the segments in the antenna. For example, in Sicily there are four coleopter species of the genus Scaurus, the Scaurus striatus can be distinguished by its ribs on elytra. Furthermore, if we consider that only the class Insecta, the most numerous class of arthropods, includes more than 1.000.000 of species, than it is very clear how difficult it is to identify each arthropod. At any rate a good rule is to make more photos, from different angle-shots, from above and from lateral sides. We could try to proceed in this way:

 

1) at first, we can try to recognize the class (see the guide) and the order (for the insects see the specific section).

2) at second, we can compare our photo with those found in the web or in books; this step could help us to identify the species or could help us to get an idea about the family or the genus.

3) finally, we can use a list of morphological characters, obviously considering the characters well visible, in order to identify the family (or the genus, or the species); we can also use this list to test the result of the previous step (for the insects see the specific section).

 

There are only a few and partial list of  morphological characters (generally in the for of "keys") available in the web which could help us to identify the genus or species. A complete documentation can be found in libraries, but we must have enough time to read it. A good aid comes from the "check lists" (lists of insects living in a specific geographical area)  because they help us to narrow the search range. For example, if we took a photo of a coleopter in Sicily, we can reduce the search to the keys and photos of the families (or genus, or species) living in Sicily. In conclusion it is difficult, but not impossible: it requires time and patience. Of course, the best satisfaction is when we made it without catching and maltreating insects.

 

 

Metamorphosis

 

The metamorphosis is a complex process through which comes the growth of many animals and mainly of insects. Not all of insects are subject to this process. Zygentoma, for example, are not subject to metamorphosis, but during their life they molt many times, ["ametabolous" insects (1)] Generally we individualize the following phases: egg, larva, pupa (or chrysalid) and adult (or imago). The metamorphosis process is called incomplete if the pupa stage (in the meaning of phase preceding adult phase in which insect doesn't eat; notice that some authors use the word nymph to indicate the phase in object instead of the larva of insects with incomplete metamorphosis) is missing. In the phase of pupa the internal tissues are usually dissolved (histolysis). When metamorphosis is incomplete, "hemimetabolous" insects (2), the larva (usually called as nymph) looks roughly like the adult arthropods, but only the last is sexually mature and, in the case of winged insects, has also completely developed wings. When metamorphosis is complete, "holometabolous" insects (3), instead the larva has a very different look. At any case, generally the larva goes through 4-5 molts, in which it abandon its old cuticle (the new cuticle requires a bit of time to become hard so the insect just moulted has a pale soft body); the stage between a molt and the next one is called "instar". There are also insects with more than one intermediate stages, in that case the metamorphosis is called complex; particularly Ephemeroptera, with some exception, have also a stage of subimago; the subimago is winged and precedes the true adult stage.

 

The entire process, that is from the egg to the to adult stage, usually takes two/eight months and moreover immature stages are longer than the adult one (for example some periodical cicadas require 17 years to become adults); sometimes the environmental condition can slow or speed up the development (some Cerambycidae and Buprestidae species were reported to take to 40-50 years to develop in dry wood). Adults usually live less than a year (Ephemeroptera and males of Coccoidea and Strepsiptera only few hours), but in some species the adult stage can last many years: some Carabidae species and Thysanura live about 2-3 years, queens of bees about 5 years, queens of ants 5 or more years (some specimens were reported to live up to 29 years), queens of some Isoptera species about 50 years.

 

 

 

picture

larva (caterpillar), pupa (chrysalid) and adult of different species of Pieris

(holometabolous insect)

picture

nymph and adult of Nezara viridula

(hemimetabolous insect)

picture

larva, pupa and adult of Coccinella

septempunctata

(holometabolous insect)

 

 

The metamorphosis process have always enchanted the man. The caterpillar that becomes a butterfly is a strong evidence of the becoming. The butterfly is not a new insect, though morphological very different, it is the caterpillar, nay it is the caterpillar in act because it is the actual form of its potentialities, just like the caterpillar is the butterfly in power because it is not explicated yet. To speak of becoming "it is necessary that there is a substratum changing... [but moreover there must be] something that remain (Aristotle, Metaphysic, chapter XII,2)", otherwise "we have not a mutation, but simply a substitution. [Moreover it is] required a dimension or an element of authentic originality between the initial condition and the final one [and this originality must necessarily] rise from the same reality that becomes, which must have had it in same way. It could not have it in «act»... therefore it must have had it in power. [It must be clear that] it is not the power that changes in act... [but it is]  the actual realty...which explicates its potentialities (A. Alessi, Sui Sentieri dell'Assoluto, Roma 1997, pp. 184-185)".

 

 

 

(1) Collembola, Protura, Diplura, Thysanura.

 

(2) Anoplura, Blattaria, Dermaptera, Embioptera, Ephemeroptera, Heteroptera, Homoptera, Isoptera, Mallophaga, Mantodea, Odonata, Orthoptera, Phasmida, Plecoptera, Psocoptera, Thysanoptera.

 

Some authors use the term "hemimetabolous" speaking only of those insects with aquatic larvae and terrestrial adults (Odonata, Plecoptera, Ephemeroptera) and the term "paurometabolous" for the other ones; for these they use also the term "gradual metamorphosis".

 

(3) Coleoptera, Diptera, Hymenoptera, Lepidoptera, Mecoptera, Neuroptera, Siphonaptera, Strepsiptera, Trichoptera.

 

Bibliography


 

 

 

text and photos by Alessandro Strano