|Checklist of the Collembola: Schematic Model of the Evolution of the Furcal Muscles|
Frans Janssens, Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium
The evolution of the furcal muscles has impacted manifestly the general body shape in Collembola. The Poduromorpha are linear forms with reduced furcal muscles. The "Entomobryomorpha" are linear forms with well developed furcal muscles, realised in two schemes: 1. tomoceromorph linear forms with elongated third abdominal segment, and 2. euentomobryomorph comma-shaped forms with elongated fourth abdominal segment. Symphypleona are subglobular forms with well developed furcal muscles. Neelipleona are subglobular forms with reduced abdomen and are a remnant sistergroup of Euentomobryomorpha. Poduromorpha, such as Neanuridae, Onychiuridae and Tullbergiidae, with vestigial furcula, and Euentomobryomorpha, such as Microfalculidae and some Isotomidae, with a furcula are secondary regressive forms. The monospecific Mackenziellidae, Symphypleona with a furcula, is a remnant of a transitive form between Poduromorpha and Neocollembola.
Poduromorpha. The Poduromorpha are characterised by: 1. the protergite being distinctly present, and 2. springing muscles situated in the fourth abdominal segment only (see fig. Pod.), and 3. secondarily an elongated and segmented habitus. The Poduromorpha are traditionally considered as being plesiomorphic (after Thibaud, Schulz & da Gama Assalino, 2004:3). In this work, they are considered apomorphic descendants of the protocollembolan ancestor having reduced springing muscles (see below).
Neocollembola. The "Entomobryomorpha", Neelipleona, and Symphypleona
form a monophyletic grouping, the 'true' springtails with well developed furca,
here called tentatively the Neocollembola (Nc in fig.1).
The Neocollembola, sister group of the Poduromorpha, are characterised by: 1. an absent protergite, and 2. springing muscles situated in the third and fourth abdominal segments (see fig. Neo.).
Xiong & al. (2008) confirm the findings of D'Haese (2002) that the
superfamily Tomoceroidea is monophyletic.
Tomoceromorpha are characterised by: 1. an absent protergite, and 2. enforced springing muscles mainly situated in the third abdominal segment, due to the elongation of the third abdominal segment (see fig. Tom.).
Panentomobryomorpha. The sister group of Tomoceromorpha, tentatively called the Panentomobryomorpha (Fig. Tree:Pe), are characterised by: 1. an absent protergite, and 2. enforced springing muscles situated in the third and fourth abdominal segment, due to the elongation of the fourth abdominal segment (see fig. Pan.).
Symphypleona. The Symphypleona are characterised by: 1. an absent protergite, 2. enforced springing muscles situated in the third and fourth abdominal segment, due to the enlarged fourth abdominal segment, 3. a ventral tube having two long eversible vesicles (see Fig. 2), and 4. secondarily a subglobular habitus.
Euentomobryomorpha. The Euentomobryomorpha are characterised by: 1. an absent protergite, 2. enforced springing muscles situated in the third and fourth abdominal segment, due to the enlarged fourth abdominal segment, 3. a ventral tube having two short eversible vesicles, which is considered plesiomorphic, since it is also found in Poduromorpha and Tomoceromorpha, and 4. a smooth midgut.
The Neelipleona are characterised by:
1. an absent protergite,
2. enforced springing muscles situated in the third and fourth
abdominal segment, due to the enlarged fourth abdominal segment,
3. a ventral tube having two short eversible vesicles,
4. a diverticulate midgut, and
5. secundarily a subglobular habitus.
At the moment of docking the furca in the retinaculum, the mesoabdominal antero-posterior tension is maximum. The flexor tension creates a downwards momentum on the posterior mesoabdominal segment. Evolutionary, to compensate for this tension, the mesoabdominal terga tend to enlarge while the mesoabdominal sterna tend to reduce. The elongation of the mesoabdominal terga has been realised differently in Tomoceromorpha and Panentomobryomorpha. In Tomoceromorpha, the anterior mesoabdominal tergum, the third abdominal tergum, is enlarged, while in Panentomobryomorpha, the posterior mesoabdominal tergum, the fourth abdominal tergum, is enlarged.
Due to the reduction of the ancestral leg of the third abdominal segment the
associated promotor and remotor muscles became obsolete.
In the Neocollembola, the muscles are reconnected to the furcal base to
function as secondary intersegmental furcal flexor and extensor.
In this scheme, the furca has the capability to evolve larger.
+---------------- Poduromorpha | <-pC-+ +----------- Tomoceromorpha +-Nc-+ | +-- Euentomobryomorpha | +---+ | | +-- Neelipleona +-Pe-+ +------ Symphypleona
This ordinal tree (Fig. Tree) is compiled
from the views of relationships among
orders of Collembola based on phylogenies proposed by Cassagnau (1971),
Massoud (1971, 1976) Moen & Ellis (1984), Bretfeld (1986), Fjellberg (1994),
Soto-Adames (1996), D'Haese (2002, 2003), Park (2002), Deharveng (2004),
and Xiong & al. (2008).
Traditionally, the Collembola have been divided into basic groups
such as Poduromorpha, Neelipleona, Entomobryomorpha, and Symphypleona,
which different authors have considered to represent orders, sections or every
category in between these two.
More recently, D'Haese (2002) and Xiong & al. (2008) have concluded that
Entomobryomorpha is paraphyletic and D'Haese (2002:1148) proposed
Tomoceromorpha as a new basic group of Collembola.
The rather straightforward ordinal phylogeny that is proposed here is based on a dichotomic phylogeny of the progressively increasing function of the furca. It is assumed that in the protocollembola the furca is not well developed. Initially, it is small: a furcula. In the Poduromorpha this small furca reduced even further in size in stead of that it developed. In some species it even disappeared. However, others specialised in increasing the furcula: the 'true' springtails, the Neocollembola, characterised with a vestigial protergite. The larger furca has an effect on the overall shape of the body: it becomes dorso-ventrally curved due to the large antero-posterior muscles that spring-load the furca. The larger furca is realised according to two different bodyplans. In the first scheme, the larger furcal flexing muscles are mainly accomodated by the third abdominal segment. In these Tomoceromorpha, the body curvature is minimal. In the second scheme, the Panentomobryomorpha, the larger furcal flexing muscles are accomodated by both the third and the fourth abdominal segment. In the Euentomobryomorpha, the curvature is typically resulting in a comma-shaped lateral habitus. Note that as a derived character, in the Euentomobryomorpha, the furca can be reduced up to absent, reducing correspondingly the body curvature. The Neelipleona with a strongly curved body are here considered as a derived sidebranch of the Euentomobryomorpha. In the Symphypleona the body curvature reaches its maximum development: the last thoracic segments and first four abdominal segments are curved upwards to such an extent that they resemble the shape of an inverse 'U', reducing effectively the body length.