Drawing setae : a GNU way for digital scienti c illustrations

In a recent paper (2015) I proposed a method to draw accurate line drawings for taxonomic studies, using taxa from Oniscidea. To complement that work, this short communication proposes a free-hand way to quickly draw areas with small setae or hairs, such as in penicils of mouth parts. Th is method enhances the previous drawing procedure, it takes a brief practice time, and allows a bett er quality of scientifi c illustrations. KeY WORDs GIMP, crustaceans, Isopoda, Oniscidea, taxonomy, drawings, setae. Crustaceans show a wide range of cuticular projections that are ordinarily called setae, although many terms indicate the same structures: sensilla, bristles or even ‘hairs’. Th ere are also some other defi nitions for these structures and they are generally meant to be homology-based (Garm, 2004). As a matt er of fact, setae can be divided into diff erent types according to their mechanical functions. Garm (2004) defi ned setae based on their mechanical functions rather than homology. Crustacean taxonomists, as well as other arthropod taxa specialists, have devoted considerable time of their valuable work in drawings to illustrate species. During the last decade, Dr. Charles O. Coleman (Museum für Naturkunde, Berlin, Germany) strongly contributed on improving methods and techniques used in this interesting fi eld of taxonomy (Coleman, 2003; 2006; 2009). I have recently proposed a new method to speed up the process of digital illustration and to produce accurate line drawings starting from pencil drawings or microphotographs. Th is method (Montesanto, 2015), exclusively based on raster graphics, uses a completely free soft ware (GNU Image Manipulation Program, GIMP). It is ordinarily used by myself and other colleagues to illustrate species (e.g. fi gures 1–5 in Montesanto et al., 2011; fi gures 6, 7, and 9 in Montesanto et al., 2012; fi gure 2 in Agodi et al., 2015) and other fi gures, such as maps, dendrograms, graphs and diagrams (e.g. Medini-Bouaziz et al., 2006; Montesanto et al., 2007; 2008; 2014; Messina et al., 2011; 2012; 2014; Lupett i et al., 2013).

Crustaceans show a wide range of cuticular projections that are ordinarily called setae, although many terms indicate the same structures: sensilla, bristles or even 'hairs' .Th ere are also some other defi nitions for these structures and they are generally meant to be homology-based (Garm, 2004).As a matt er of fact, setae can be divided into diff erent types according to their mechanical functions.Garm (2004) defi ned setae based on their mechanical functions rather than homology.
Crustacean taxonomists, as well as other arthropod taxa specialists, have devoted considerable time of their valuable work in drawings to illustrate species.During the last decade, Dr. Charles O. Coleman (Museum für Naturkunde, Berlin, Germany) strongly contributed on improving methods and techniques used in this interesting fi eld of taxonomy (Coleman, 2003;2006;2009).
I have recently proposed a new method to speed up the process of digital illustration and to produce accurate line drawings starting from pencil drawings or microphotographs.Th is method (Montesanto, 2015), exclusively based on raster graphics, uses a completely free soft ware (GNU Image Manipulation Program, GIMP).It is ordinarily used by myself and other colleagues to illustrate species (e.g.fi gures 1-5 in Montesanto et al., 2011;fi gures 6, 7, and 9 in Montesanto et al., 2012; fi gure 2 in Agodi et al., 2015) and other fi gures, such as maps, dendrograms, graphs and diagrams (e.g.Medini-Bouaziz et al., 2006;Montesanto et al., 2007;2008;2014;Messina et al., 2011;2012;2014;Lupett i et al., 2013).
Drawing setae with GIMP Nauplius, 24: e2016017 I have mainly worked with the taxon Oniscidea (terrestrial isopods); in this group, some body parts show areas with several small setae or fine hairs.Buccal appendages, such as mandibles and maxillulae, generally bear penicils entirely covered by fi ne setae that are not easy to draw without a great waste of time.
Th erefore, to complement the previously cited work (Montesanto, 2015), I here propose a free-hand way to quickly draw small setae with a single movement of the mouse.

MeTHOD
GIMP version 2.8.14 on a Mac Mini, late 2013, with Mac OS 10.11.5, has been used.GIMP workspace and sett ings are the same as described in Montesanto (2015), as shown in Fig. 1.
Once the fi le is opened, the sett ings in the Tool Options window of the Paintbrush Tool need to be changed in order to set the brush preferences (Fig. 2).

Step-by-step instructions
In order to easily draw a perfect seta with a natural appearance, the user needs to add a dynamic to the brush.Dynamics are quite oft en used by artists who work with digitizer boards or digital graphic pens.In our case a basic dynamic will be added to a simple mouse movement, with the following instructions: • choose the Paintbrush Tool in the Toolbox (Fig. 1F); • choose the appropriate brush: N. 2. Hardness 075 (the same in Montesanto, 2015); • left click on the "Dynamics" butt on in the Tool Options panel (Fig. 2A); • choose "Basic Dynamics" from the menu; • in the "Fade Options" bar, choose a value from 50-90, depending of the length of the seta that should be drawn (see aft er) (Fig. 2B); • set the "Reverse" option as active (Fig. 2C); • the remaining options can be left with default values (as shown in Fig. 2).
How to draw a small seta Each seta is drawn by pressing the left mouse butt on and, simultaneously, moving fast the mouse toward the direction of seta tip.Th e mouse butt on is to be released in the exact point where the seta tip should be drawn.It's important to move the mouse quite quickly.If the movement is too slow the line will just appear as a shade of grey but with no smaller dimension in the apex.If it is suffi ciently fast the user can also observe how the tip tends to be reduced in size (Fig. 3A, B).Aft er the fi rst att empts it will become a natural and repetitive movement, and the right balance between the speed of mouse and the fade length will be easily found.
Fade Options is particularly important as it determines the length of the fading and therefore the actual length of the seta; its base width is the brush size, which should follow the line of the existing image.Th us, a line drawing made with a 6 pixels brush, should have setae drawn with a smaller brush (e.g.size: 3 pixels).Usually, a brush size of 3-5 pixels would be the right choice for most of the line drawings.An example of setae drawn with diff erent brush size is reported in Fig. 3C.It is also important to draw setae using the maximum viewing zoom level; good examples are 400% or even bett er 800%.Th en, image visualization can be shift ed on the screen by pressing the space bar on the keyboard and dragging the mouse.Examples of drawings of setose (or hairy) areas in isopod appendages using the present technique are shown in Fig. 4.   Further information could be retrieved in the GIMP documentation website (htt ps://docs.gimp.org/),or asked directly to the author.

DIscUssIOn
In his short paper, Coleman (2009) commented that the free-hand method is one of three possible ways to draw setae.As reported by the author, the advantage is that it is possible to draw every shape of seta or spine; but the disadvantages are the waste of time and the imperfect lines, especially for the tip of the seta.Other details were also shown in Coleman (2003).
In my opinion, the presented method brings an enhancement to the previous drawing procedure (Montesanto, 2015).Figure 5 clearly shows diff erences in drawing setae with the present method.Stronger and/or single setae should be drawn following the technique in Montesanto (2015).
This method is very easy to use, takes little practice time, and allows a bett er quality of scientifi c illustrations.Moreover, it is completely free as it uses open source soft ware and commonly used devices.Further details (e.g. about saving the fi les or exporting the plates) may be found in Montesanto (2015) or directly requested to the author.

Figure 2 .
Figure 2. Screenshot of the Tool Options window, with Paintbrush Tool activated.All the sett ings should be as shown; brush size can be modifi ed accordingly to the line drawings.

Figure 3 .
Figure 3. Visual explanation on how to draw a single small seta.A, grey circle indicates the starting point (click and keep the left mouse butt on pressed); then, move fast the mouse to the direction shown by the red dott ed line; the red circle indicates the releasing position of the mouse butt on.B, obtained result.C, examples of result using diff erent brush sizes (2-8 pixels).

Figure 4 .
Figure 4. Examples of line drawings of terrestrial isopods anatomical parts.Areas with small setae drawn with the present method are indicated with a black line.Numbers indicate brush size.A, antenna; B, right mandible; C, maxilliped; D, pereopod 1; E, telson; F, uropod.

Figure 5 .
Figure5.Example of a small setae area in a terrestrial isopod species: penicils of a maxillula inner branch.A, drawn with Montesanto method (2015); B, drawn with the method presented here.Brush size is the same (3 pixels) but setae appear more realistic with the second method.

AcKnOWLeDGeMenTsI
would like to express my very great appreciation to Dr. Annamaria Pulina (University of Pisa, Italy) for her noteworthy contribution in the revision of English.Many thanks to Dr. Stefano Taiti (ISE-CNR, Florence, Italy) for his endless constructive comments and precious advice about drawing and preparing plates for terrestrial isopod taxonomy, and also for his precious suggestions in the early version of this paper.A special acknowledgement goes to Prof. Paula B. Araujo (Universidade Federal do Rio Grande do Sul, Porto Alegre, Brasil) for encouraging me in the proposal of this short contribution.I would like to thank Dr. Charles Oliver Coleman (Museum für Naturkunde,