Preheat the oven to 180°C (160°C fan forced). Line 2 x 12-hole muffin pans with paper cupcake cases.
Open the cake box, remove enclosed frosting sachet and reserve for another use. Put the cake mix in a large mixing bowl and add the oil, water and eggs. Whisk (by hand or with an electric mixer) for 2-3 minutes, until smooth and creamy. Divide the mixture evenly among the cupcake cases. Bake both pans together in the oven for 18-22 minutes. Remove from the oven and carefully transfer the cupcakes to a wire rack to cool completely; about 30 minutes.
Put the frosting in a large bowl and use the green food colouring paste or gel to colour the frosting bright green. Reserve 1 tablespoon of frosting. Use a small palette knife to spread the remaining frosting on the top of the cupcakes.
Using the tubes on black writing icing, pipe 4 concentric circles on each frosted cupcake. Gently draw the tip of a cocktail stick through the circles from the centre outwards at intervals to create a spider’s web pattern on each cupcake.
Use a rolling pin to flatten 24 of the pastilles into ovals. Cut each into 8 thin strips and place the strips on top of each cupcake to form the spider’s legs. Top each with a whole pastille for the spider’s body using the reserved frosting to secure in place. Using the black writing icing, pipe tiny eyes on the spiders. Serve immediately or keep in a cool place for up to 2 days.