Most of the slug properties are controlled by the tip. Regardless of the slug’s age, each cell contributes to the movement, with the tip giving the direction and the signal for movement. To model the slug movement, we used the formulated equation of the slug movement of Innouye’s work [
10] and the results of Smith’s work [
27]. There are some assumptions of slug movement in Innouye’s work [
10]:
1.
A slug does not change its speed during the movement.
2.
Migrating velocity is essential when the slug moves forward to the light and heat.
3.
All cells move in the same direction with the same speed.
4.
Each cell moves actively with the constant motive force.
5.
Each cell meets with an intrinsic resistance related to its speed
6.
The cells in the tip of the slug give the direction and the sheath keeps all cells constrained together.
7.
There is no resistance from the sheath on the sides nor on the back of the slug.
Also, in Innouye’s work [
10], we can find an equation between the slug’s length, width, and velocity. They have extracted Eq. (
1) by making a multiple regression analysis on the data of
27 different slugs.
$$\begin{aligned} \frac{1}{v }=0.27\frac{1}{L}+0.0075\frac{1}{\omega }+0.26, \end{aligned}$$
(1)
where,
v denotes the migrating velocity and
L,
\(\omega\) for length and width of the slug respectively. In this equation, the coefficient for the width of the slug is so small so that it can be regarded as zero. Thus, we can conclude the Eq. (
2) based on the length of the slug:
$$\begin{aligned} {\frac{1}{v }=0.27\frac{1}{L}+0.26}. \end{aligned}$$
(2)
As shown in Fig.
2 (green part), in the beginning, each cell can be in one of the three cell types: PST (pre-stalk), PSP (pre-spore), or Pacemaker. If it is a PSP cell, it will just follow the tip cAMP signal. However, if it is a PST cell, it will follow the whole flowchart. The PST cells notice when they become a slug (identification of emergent property), i.e., when they realise that they are surrounded by the same type of cells (they are surrounded by other PST cells). Additionally, if they are pacemaker cells, they act as PST cells, with the ability to release cAMP signals spontaneously. In this model, we have two quorum sensing phenomena, which are indicated by red circles (one and two), used for the PST cells to collectively decide on which direction to follow. Additionally, we use a diffusion-based approach based on a chemical signal (DIF) that helps PST cells inside the anterior part to measure the population size, informing in this manner the cell about the slug length, its motive force and the velocity to adopt (Eq.
2). This is used both for slugs’ merging and during phototaxis.