VI ARTHROPODA 233 



with light produced by artificial means. The emission of the light 

 appears to be under the control of the nervous system, there being 

 probably some special mechanism for controlling the access of air to the 

 light-producing cells. Physiologically the significance of these organs is 

 connected with sexual attraction. In many cases the light is not a 

 continuous glow but is emitted in long or short flashes : as is well known 

 such an intermittent light is more efficient in attracting attention than 

 is a steady glow. Further, the female sex is in some cases distinctly 

 recognizable by the longer duration of the flash. 



The sexual attraction of this naturally-produced light is merely a 

 special case of the well-known " attraction towards light " shown by 

 many insects. And this in turn rests on the fact that an insect in the 

 presence of light proceeding from one localized source tends to assume 

 an attitude in which its head is directed towards the source of light so 

 that its two eyes receive the impression equally. To put it crudely and 

 not quite accurately it " faces " towards the direction from which the 

 light-impression comes. This tendency is well brought out in Fig. 105, 

 a record made in tropical South America twenty- five years ago. The 

 author sat at work in the evening by the light of a lamp on his left 

 hand. Numerous homopterous insects alighted on the sheet of paper on 

 the table in front of him. Each insect within a few seconds of its 

 flopping down on the table rapidly adjusted its position and remained 

 stationary with its long axis pointing towards the lamp, as indicated by 

 the arrows of the diagram. It is obvious that an insect can fly only in 

 the direction in which its head points : consequently if it adjusts itself 

 during flight so that its head points towards the light it necessarily flies 

 towards the light. As regards the insect's orientating itself towards 

 the light there is in the author's opinion no scientific justification 

 for excluding the action of psychical factors as is done by some 

 physiologists. 



The excretion of nitrogenous waste products is carried out in Peripatus 

 by numerous pairs of nephridia. In the more typical arthropods the 

 continuous series of nephridia is no longer present but the last survivors 

 of the series are still distinctly recognizable in the Crustacea and the 

 Arachnida. In the more lowly organized Crustacea they form what are 

 known as the shell glands — opening at the base of the second maxilla — 

 while in the more highly developed Crustacea such as the Crayfish, 

 Lobster, or Crab, they form the green glands — opening at the base of 

 the second antenna. In the Arachnida they form what are called coxal 

 glands opening at the base of the fifth appendage. In the Insects they 

 are no longer recognizable, there being here numerous blindly ending 



