CodeIgniter -Error Call to a member function model() on a non-object

I'm creating a registering from. Whenever I try to insert data to db it throws this error.

A PHP Error was encountered Severity: Notice Message: Undefined property: insert_ctrl::$insert_model Filename: controllers/insert_ctrl.php Line Number: 38 Fatal error: Call to a member function form_insert() on a non-object in C:\xampp\htdocs\NewProject\application\controllers\insert_ctrl.php on line 38

This is my insert_ctrl.php

<?php

class insert_ctrl extends  CI_Controller{

    function _construct()
    {
        parent :: _construct();
        $this->insert_model->form_insert($data);

    }

    function index()
    {
        //Including validation library
        $this->load->library('form_validation');
        $this->form_validation->set_error_delimiters('<div class ="error">','</div>');

        //Validating name field
        $this->form_validation->set_rules('dname','Username','required|min_length[5]|max_length[15]');
        $this->form_validation->set_rules('demail','Email','required|valid_email');
        $this->form_validation->set_rules('dmobile','Mobile No.');
            $this->form_validation->set_rules('daddress','Adress','required|min_length[10]|max_length[50]');

            if($this->form_validation->run()==FALSE)
            {
                $this->load->view('insert_view');
            }
            else {
                //Setting values for db table
                $data=array(
                    'Student_Name'=>$this->input->post('dname'),
                    'Student_Email'=>$this->input->post('demail'),
                    'Student_Mobile'=>$this->input->post('dmoblie'),
                    'Student_Address'=>$this->input->post('daddress')

                );
                //Transfering data to model
                $this->insert_model->form_insert($data);
                //loading view
                $this->load->view('insert_view');
            }
        }
    }

//insert_model.php

<?php
class insert_model extends  CI_Model{
    function _construct()
    {
        parent :: _construct();


    }

    function form_insert($data)
    {
        $this->db->insert('student',$data);
    }
}
?>